This issue of mine has not been resolved yet. See this post for details.
Unless the solution could be, again, to increase the Puma Worker Timeout and Gitaly Timeouts even higher (past 5 minutes).
EDIT: Actually, I just refreshed the webpage and now a Green Merge button has appeared nearly instantaneously after the Merge Request page reloaded. So I’m going to say this issue has been resolved, but I really would like feedback on how this process could be improved.