Yes, the errors you get when you switch to XHTML are caused by Q2A's HTML tags being generated in upper-case, which is not acceptable XHTML.
The next two are trickier. If you try to tell the validator about the FB namespace you will get another error. The best solution might be to get rid of this (again in the plugin):
and replace it with this:
Alternatively, if you are using jQuery with your theme (or in future using Q2A 1.4, which provides jQuery), you might find this sort of solution appropriate: