Informal vs. formal journal acceptance letter I got a letter from a journal editor telling me that they 'would be happy to accept my paper'; the editor asked me to take care of some minor citation issues (I carelessly misspelled one author's last name and I misplaced the publication year of a cited work.) and then upload the final version of the manuscript to the submission system. Although this letter sounds very certain/positive to me. I am still a bit wary and anxious (after all, this is still not a formal acceptance letter). I quickly resubmitted the final version of the paper to the submission system in the same day. It has been two weeks since I heard from the editor and resubmitted the final file. I know that it's impossible to expect a speedy response. But, in my scenario, how long should I wait to write back to the editor and ask for a formal acceptance letter (or is it even okay/appropriate to do this?) At this stage, should I be worried (about any possible hiccups that may stand on the way of getting the paper published)? Thanks very much for your insights and experience sharing.
You shouldn't worry much.  It's extremely unlikely that an informal acceptance will turn into a rejection, although I suppose it is theoretically possible.  In my experience, this is something editors do to get leverage over authors: authors are faster and more diligent about making revisions when acceptance has not yet been guaranteed, even when they know it is almost certain.

I wouldn't worry about the two week delay or get in touch with the editor yet.  For example, if the editor is in the U.S. the Thanksgiving holiday intervened, and the end of the semester is always a busy time.  It's also possible that the editor is checking with a referee (although this may not be necessary for such minor revisions).  It can't hurt to ask whether the revised paper was received, but I'd recommend waiting another week or two.

Incidentally, asking you to add citations sounds a little odd.  Are these from a referee report, and do they look like important references that you missed?  One unethical thing editors are occasionally tempted to do is to pressure authors to add citations to manipulate citation rates.  (For example, adding citations to their journal or their friend's journal.)  That's not necessarily going on here, but if it is then it's an inappropriate use of informal acceptance to get leverage over authors.