1) Yes
2) It can be played either way, my first playthrough was lone wolf, I'm currently on my second - it's just a different experience
3) Pet Pal is mandatory if you are a completionist
4) If you don't have summons, you're going to have trouble in a lone wolf playthrough due to being CC-spammed by the enemy who only have 2 targets. You're going to need your cannon fodder.
But if you invest in willpower/bodybuilding enough + use elemental shields + invisibility as air skill (for both characters) you can get away with just 2 characters in combat at a time. You need to be careful with relying on invisibility though - poison/burning/aoe still affects you and brings you out of cloak.
5) You need both characters mages; Warrior/mage + Rogue/mage was my setup - you have MORE than enough skill points for it
6) Yeah, both characters need to be tanks in their own way so invest in elemental shields - air shield is the best IMO as it prevents you from being stunned (most dangerous CC imo - second to knockdown) and stuns enemies that hit you. Need at least 10 INT to keep those shields up throughout combat.
