1) Gaming Club offers $300-odd if you play up to $2/4 limit. You'll have to free up most of it at 2/4 and 3/6 because below this you can only get 120. Interpoker is by far the best for consistent bonus, with $90 100% match bonus EVERY single month, which is freed up quickly. NL is $100 buyin but same blinds as party 50 and poor players. Bet365 is a prima site that frees the bonus in about an hour. PokerStars gets a nice deposit bonus and I like the site in general.
2) No idea
3) UltimateBet has the nicest software IMO and is now much better to use with PT (used to be hopeless due to poor PT support). PokerStars is definitely up there, in fact one could argue it has the best software out there.
4) Prima poker sites offer the most passive players, though I'd say PokerStars is not far behind. UltimateBet is tight but features the odd maniac and a lot of passives.
5) Nothing comes close to party/empire. Only other places I've tried are UB and PokerStars, with UB offering the easiest use now there's numerous patches to help with this. Check out the PT website.