For 2) I'd like to hear some more thoughts.
One suggestion is to put up a huge warning sign when you try to resign in 3P. "This will destroy the game for your opponents! do you really want to resign a 3 player game? It's considered bad sportsmanship". Personally I agree with those sentences, but wouldn't want to put them into the client unless it's already the opinion of a large percentage of the community.
I feel/fear that it would not help (a lot).
Another thought on resigning: if a player is taking too long (for whatever reason), he can be made to resign. If I am not mistaken, only one opponent needs to click yes and thus the player with the most VPs can take the win even if he has a poor engine.