Your theory about REBECCA sounds very interesting and seems to make a lot of sense. As I am especially interested in Hebrew names (and even more bacause my third daughter is called Rebecca) I would like to know where you got this information from. I am always looking for reliable books in that respect.
The verbal root ravak (resh-beth-kuf) does not appear in the Hebrew Bible (ravak there means "flash, lightning"), so I suspect it is Ivrit (unfortunately I don't have my Ivrit dictionary with me). Is that the case?
Martin Noth doesn't say much about the name Rebecca, he does make a connection to an Ivrit noun, though, that has to do with "to tie." The Encyclopedia Judaica doesn't say anything of the name either (with some names it does).
So I am really anxious to learn more.
By the way, the meaning "fat" or "well-fed" for MIRIAM has nothing to do with Rebecca, this is just coincidence.