Debian apparently has a separate repository for debug packages.  I found and installed nemo-dbgsym and nemo-fileroller-dbgsym, and re-ran gdb.
(terminal output for gdb without dbgsym is shown in comment #6, above)

No crash.

Ran without gdb.

Crash.

I will attach the no-crash terminal output for gdb nemo.

I find it significant that both times it crashed, whether with gdb without dbgsym, or without gdb, the same seven lines were printed:

with gdb (no dbgsym):
(nemo:2004): Gtk-CRITICAL **: 20:42:29.116: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:2004): Gtk-CRITICAL **: 20:42:29.117: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:2004): Gtk-CRITICAL **: 20:42:29.117: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:2004): Gtk-CRITICAL **: 20:42:29.118: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:2004): Gtk-CRITICAL **: 20:42:29.118: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:2004): Gtk-CRITICAL **: 20:42:29.118: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:2004): Gtk-CRITICAL **: 20:42:29.118: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     


no gdb:

(nemo:1831): Gtk-CRITICAL **: 19:45:52.537: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:1831): Gtk-CRITICAL **: 19:45:52.537: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:1831): Gtk-CRITICAL **: 19:45:52.537: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:1831): Gtk-CRITICAL **: 19:45:52.538: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:1831): Gtk-CRITICAL **: 19:45:52.538: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:1831): Gtk-CRITICAL **: 19:45:52.538: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     
                                                                                                                                                                                                                                                                                                                                                                                                                                       
(nemo:1831): Gtk-CRITICAL **: 19:45:52.538: gtk_container_foreach: assertion 'GTK_IS_CONTAINER (container)' failed                                                                                                                                                                                                                                                                                                                     

...but were absent when nemo was run with gdb with dbgsym.