One other method to quickly exit insert mode if you can't remap keys is ^C. I did not know this even though I used gvim on Windows computers not under my control for years.
<Esc> or CTRL-[ End insert or Replace mode, go back to Normal mode. Finish abbreviation.
CTRL-C Quit insert mode, go back to Normal mode. Do not check for abbreviations. Does not trigger the InsertLeave autocommand event.