I have 11679 as branch lp988601, is the problem patch in or out of that version?  setAttribute() is still causing problems, so I  am guessing no.   But I could not apply the patch from (5) successfully:

 patch -p0 </tmp/1043571v2.patch
patching file src/attribute-rel-util.cpp
Reversed (or previously applied) patch detected!  Assume -R? [n] 
Apply anyway? [n] y
Hunk #1 FAILED at 128.
Hunk #2 FAILED at 147.
Hunk #3 FAILED at 155.
3 out of 3 hunks FAILED -- saving rejects to file src/attribute-rel-util.cpp.rej
patching file src/attribute-rel-util.h
Reversed (or previously applied) patch detected!  Assume -R? [n] 
etc.

In any case, 11679/lp988601 has other issues which do not fall on setAttribute lines, and there does not seem to be anything in the repr-css.cpp part of 1043571v2.patch which addresses them (possibly addressed elsewhere in the patch though):

1.  just after the comment containing

    * HACK for now is to strip off em and ex units and add them back at the end

there is a piece of code that does not check that l>2 before using strncmp, which spammed the valgrind output
with 40 something invalid reads.  It should be:

    int l = strlen(value_unquoted);
    if (l>2 &&
            (!strncmp(&value_unquoted[l-2], "em", 2) ||
             !strncmp(&value_unquoted[l-2], "ex", 2)
            )) {
        units = g_strndup(&value_unquoted[l-2], 2);
        value_unquoted = g_strndup(value_unquoted, l-2);
    }

Also, please have mercy  and don't use "l" for a variable, it is nearly indistinguishable from "1", especially if your eyes are as
bad as mine.

2.   (many) leak(s) at line 343
    dst->mergeFrom(src, "");

here is one

==5076== 160 (128 direct, 32 indirect) bytes in 8 blocks are definitely lost in loss record 51,144 of 58,146
==5076==    at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==5076==    by 0x834A9B2: Inkscape::XML::SimpleNode::setAttribute(char const*, char const*, bool) (gc-core.h:74)
==5076==    by 0x8347CAD: Inkscape::XML::SimpleNode::mergeFrom(Inkscape::XML::Node const*, char const*) (simple-node.cpp:660)
==5076==    by 0x833D814: sp_repr_css_merge(SPCSSAttr*, SPCSSAttr*) (repr-css.cpp:343)
==5076==    by 0x8B08327: ???


3.  1 leak at 351  (6200 bytes in the test run)

    guchar *const str_value_unsigned = cr_term_to_string(decl->value);

when called from 410

    sp_repr_css_merge_from_decl(css, decl_list);

==5076== 6,200 bytes in 25 blocks are possibly lost in loss record 57,124 of 58,146
==5076==    at 0x402A420: memalign (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==5076==    by 0x402A4DE: posix_memalign (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==5076==    by 0x52FF2E1: ??? (in /lib/i386-linux-gnu/libglib-2.0.so.0.3200.3)
==5076==    by 0x534B73F: g_slice_alloc (in /lib/i386-linux-gnu/libglib-2.0.so.0.3200.3)
==5076==    by 0x534EEE9: g_string_sized_new (in /lib/i386-linux-gnu/libglib-2.0.so.0.3200.3)
==5076==    by 0x534F526: g_string_new (in /lib/i386-linux-gnu/libglib-2.0.so.0.3200.3)
==5076==    by 0x86CE81E: cr_term_to_string (cr-term.c:287)
==5076==    by 0x833DD09: _ZL27sp_repr_css_merge_from_declP9SPCSSAttrPK14_CRDeclaration.isra.27 (repr-css.cpp:351)
==5076==    by 0x833E77B: sp_repr_css_attr_add_from_string(SPCSSAttr*, char const*) (repr-css.cpp:410)
==5076==    by 0x818C837: sp_css_attr_from_style(SPStyle const*, unsigned int) (style.cpp:4516)
==5076==    by 0x86A607C: MarkerComboBox::create_marker_image(unsigned int, char const*, SPDocument*, Inkscape::Drawing&, unsigned int) (stroke-marker-selector.cpp:493)
==5076==    by 0x86A6CF2: MarkerComboBox::add_markers(_GSList*, SPDocument*, int) (stroke-marker-selector.cpp:404)



5. a bunch of leaks at 43
    : SimpleNode(g_quark_from_static_string("css"), doc) {}

This last one drops a lot of memory:

==5076== 25,239 (400 direct, 24,839 indirect) bytes in 25 blocks are definitely lost in loss record 57,757 of 58,146
==5076==    at 0x402BE68: malloc (in /usr/lib/valgrind/vgpreload_memcheck-x86-linux.so)
==5076==    by 0x865D205: Inkscape::XML::CompositeNodeObserver::add(Inkscape::XML::NodeObserver&) (gc-core.h:74)
==5076==    by 0x8348521: Inkscape::XML::SimpleNode::SimpleNode(int, Inkscape::XML::Document*) (simple-node.cpp:178)
==5076==    by 0x833D022: sp_repr_css_attr_new() (repr-css.cpp:43)
==5076==    by 0x818C827: sp_css_attr_from_style(SPStyle const*, unsigned int) (style.cpp:4515)
==5076==    by 0x86A607C: MarkerComboBox::create_marker_image(unsigned int, char const*, SPDocument*, Inkscape::Drawing&, unsigned int) (stroke-marker-selector.cpp:493)
==5076==    by 0x86A6CF2: MarkerComboBox::add_markers(_GSList*, SPDocument*, int) (stroke-marker-selector.cpp:404)
==5076==    by 0x86A71F6: MarkerComboBox::sp_marker_list_from_doc(SPDocument*, int) (stroke-marker-selector.cpp:318)
==5076==    by 0x86A7834: MarkerComboBox::init_combo() (stroke-marker-selector.cpp:193)
==5076==    by 0x86A8285: MarkerComboBox::MarkerComboBox(char const*, int) (stroke-marker-selector.cpp:74)
==5076==    by 0xA387CFF: ???