1) Yes.
2) No, you have to use the kernel provided by Linode (2.4 or 2.6). They will include requested features if you ask nicely (and give a justification) but most things are already in.
3) Yes.
4) Yes.
5) The Planet (Dallas) filters some ports (see
here). Hurricane Electric (Fremont) does not filter anything, afaik (my Linode is there). Linode do some layer 2 and layer 3 filtering to stop nosey/naughty people screwing with their network. The host only passes your traffic to your UML instance.
hth