Looks like these can use a generic callback.

and again - Looks like these can use a generic callback.

WM_DRAG_*** isn't private, so would prefer just to have UI_but_drag_set(uiBut *but, PointerRNA *ptr, int dragtype) which takes the WM_DRAG_*** argument too. Then it can be used anywhere.

Would suggest something like

void UI_but_drag_set_rna_type(uiBut *but, int type, PointerRNA *ptr)
{
  but->dragtype = type;
  but->dragpoin = (void *)ptr;
}

If the suggestion above is followed this seems to be really couple of lines only?

Same as above.

I dont see why this would leak memory, can the comment be removed?

It works with Python, but not RNA path resolution.

This should use " quotes, nowhere else uses single quotes in RNA.

This shouldn't be some unknown question, run blender with --debug and it will print unfreed blocks.