Comment 6 for bug 1765401

I don't have the solution for your issue unfortunately. But I can eliminate a few elements:
Gnucash has switched to gsettings/dconf starting with 2.6.0. That means gconftool and gconftool2 changes are ignored by gnucash. As you discovered making the change in dconf is seen by gnucash 2.6.x.

As I mentioned on the gnucash mailing list, gtk as of 3.10 has completely removed user control over the tool bar style. It is up to the developers to handle it (which hasn't happened for gnucash so far). That means unfortunately I don't know of any way to reliably control this setting in gnucash 3.x.

From the test result I infer gtk-query-settings only queries settings for gtk3, so any result you get from that will not help.

The list of files you have tried didn't include
$HOME/.config/gtkrc-2.0
My own experimentation shows settings in this file take precedence over
$HOME/.gtkrc-2.0

In addition you may check for the presence of
/etc/gtk-2.0/gtkrc
Which may also set this as well as the gtkrc file for the current gtk theme Bionic is using.

Perhaps with the switch back to gnome shell Ubuntu has chosen to make a very strict theme to enforce their style. But I have no idea about this as I'm not on Ubuntu.