Informal vs. formal journal acceptance letter
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.