1. I can't speak of other servers, but I'm happy with population on Artemis.
2. Plenty of legit combos, and plenty that are gimped. P/K is one of the better dedicated healers, but no offense to speak of. W/K well geared is a terror in siege, but not a top instance tank (might pull it off sometimes, but you won't be the best). K/P is a top instance tank, but poor offense capabilities. P/W & W/P I basically never see - probably gimped.
3. I haven't seen a truly fearsome scout in siege in a long time. Rogues and P/S (Iceblader) are the scariest usually. There are others that can dominate occasionally when well played (W/K, M/K come to mind), but not as common.
@Auros: I play a Ch/R in siege (relatively well geared, ~2k siege points guild). It's like a knight (tank), but with more offense. Even at their best I'd expect a champ less to be formidable in PvP than Rogue or W/K (and not sure on Wardens). Haven't met a good warlock in siege yet.