You wouldn't be able to instigate a conversation, but you can reply to any existing messages. (as I said... limited functionality)

I would think not because when you set up a jabber a/c via those extra plugins it will be a telepathy-gabble a/c and you want telepathy-haze. You'll have to do this one manually.

I'm a little hesitant to write up a howto as this really isn't a fix, I probably wouldn't even class it as a work-around. The issue isn't with the clients but with the xmpp servers at facebook. They've said it is going to be fixed some time soon, so I'd recommend just sit tight for a while.

If you do wish to muck about with it I used http://talk.maemo.org/showthread.php?t=80501 as a guide.