Glad you got partly solved, at least! I don't have any flac, so I can't help you there - I'm sure there are plugins for quicktime though, which I think is what Amarok uses.

I suspect having things installed from different sources could well cause problems, although you might be able to keep them separate by tweaking installation directories. Probably better asking that one in one of the kde-mac support places - I think there's an IRC channel on irc.freenode.org, and there's probably a mailing list!

You might be able to find standalone packages for Kate somewhere, like the Amarok ones.