Don - you just have find words fascinating & looking them up irresistible! ;-)

Black belt - Nope, sorry. You'll find some very peculiar claims about the origins of butterfly on the net. And odd claims about the relationship to flutterby as well. Butterfly goes all the way back to A/S & there are similar forms in related languages such as Old Dutch. It really is simply butter + fly, the most likely explanation of origin being that several common species have creamy or yellow wings.

If Shakespeare referred to butterflies as anything else, the only other word I can think of as likely would be atomy which, related to atom, simply means any tiny creature (insect, almost certainly). But I would have thought butterflies were a bit big to be classed as an atomy. As for flutterby, I can't find any evidence of its first usage, nor that it's anything other than a simple play on the original word. Flutter, outside of its regular meaning, can refer to gambling of course, but there's also a meaning of indulging in sexual activity. Both usages date to the mid-19thC, so my guess would be that flutterby is no earlier than that at best.