I can edit my signature here without a hang/error/problem, long as I do not try to cut/copy/paste anywhere. I haven't experienced the beachball hang, but I _have_ seen portions of edited text in a message vanish along with its signature when selecting "none" from signature popup (in an email with a signature).
I have two signatures, an old one and a newer version that isn't much different (nothing flashy, no color or logo), but I can choose either one. Granted, none of this should be a problem and we shouldn't need to have this discussion(!), but it does suggest the possibility of having multiple pre-designed signatures in that popup menu. You can name them anything you like, and as long as it's the last thing you do before hitting "send," it might be a viable workaround (albeit, one that may not suit you).
Best guess I have is that the problem has something to do with how things are stored in the clipboard. (If you cut or copy something in an email, then switch to the Finder > Edit menu > Show Clipboard, you'll see it there.) It may be that - somehow, for some reason - a 'return' is added to the cut/copy that forces a new line in an email when you then choose to paste. Or perhaps Mail is expecting some format/color/spec data that gets stripped in the clipboard. Since a similar problem occurs in other apps, it kinda suggests an issue beyond just the Mail.app.
As for a response from Apple....? A bug fix buried in some update (someday) is about the best we can hope for. This forum is a great resource for all, especially since - let's face it - we're all beta testers now. But I don't think Apple is in the habit of directly answering questions/issues here.