You have to declare those two structs in start of the file too (like Mesh etc.), this breaks gcc too.

Should be static (triggers warning under linux/gcc).

Declarations of vars after code - errors in gcc too (C90 forbids this ;) ).

Picky: better to try to avoid such meaning-less changes in patches. ;)

indices, not indeces.

Might be a matter of taste, but would rather deduplicate it this way (not so much important here, but below with longer code saves quite a bit of lines):

if (ese->htype == BM_VERT) {
    BMVert *v = (BMVert *)ese->ele;
    const int idx = derived_index_map ? derived_index_map[BM_elem_index_get(v)] : BM_elem_index_get(v);
    copy_v3_v3(cent, dm_verts[idx].co);
}

The same goes for similar cases below.

Here again, you can deduplicate with a mere:

const int idx = vertexmap ?  vertexmap[BM_elem_index_get(emv)] : BM_elem_index_get(emv);

Again, this can be done in one line:

const int derived_index = index_map ? index_map[edit_vert_index] : edit_vert_index;

As a side note, might be good to have a consistent naming for that index_map stuff. ;)