Stock connector is not listed anywhere in the trust center, but keeps on popping up.
I had the exact same problems as you, with the exact same add-in. I just spent like 4 hours trying to fix it and I think I fixed it. this is what worked for me:
open excel. click "File", click "account", the click "switch account". it turns out I can login to office with two difference email addresses and I had to switch to the second email address. and then i deleted the add-in in one of the standard ways that everyone was telling us and that worked. that did it for me. i really hope it works for you too.
to clarify: my office 365 subscription is tied was tied to one email address, but excel still lets you login under various user email addresses. and that was the source of the whole problem: that i have TWO user accounts that use excel under this sub. and i was trying to remove the add in whilst logged in under only one of those email addresses. give it a shot.
even though i finally succeeded, this is outrageous how unclear and nearly-impossible it is to remove these add ins.