Ahhhh.... so refreshing to have this working now! Thanks so much for the tip Mose. It took me a while to implement it since I am not too fluent with the file system in AuCTeX and needed to figure out where to find the file ("sudo find / -iname "beamer.el"). Fixing this was easier than I expected, but I could not anticipate when I got your e-mail in what was a pretty busy week.
If you (or anyone else in the list) happens to have time, I would appreciate knowing how can one know in a more general fashion what controls whether AUCTeX prompts for optional arguments or not.