1) The level will scale to whoever made the group. So if you're far above them, you won't get much.
2) I focused on Overshield and HP, ignored resistances, I just avoid being hit, by anything, ever. (Necromancer)
3) All should be doing some damage, whichever ailment actually procs will be the one visible to you.
4) No, there are discussions floating around about how things scale, because when you do the math on my particular build, I should have almost no life and no shield, but I have a massive pool of HP and tons of shield.
5) Damn.

1. It depends on what you are doing. If it is just story, I am not sure. Expeditions, or other end game things, scale to whatever it is set to which does not matter about your level.

2. Resistance is very important if you get enough of it. Example I have around 2k of every resist which is 40% or so reduction from all types of those damages. However resistance alone is not enough, you need a nice health pool/overshield pool to work with too once you get to higher tier end game. I only had 12k hp and was doing champ 70s before I needed to increase my defence to health. Also overshield is a noob trap unless you are going into something that supports refilling it either skill wise or node wise, end game that is.

3. It will add all of that damage when hit, but only your highest damage will be the one to trigger its ailment. You can increase this on the gates of fate in a node to make skills do 2 , which will take their 2 highest damage types instead of highest. So if a skill does fire damage and you have higher frost damage coming from the skill than fire, it will actually not trigger burning, but freeze.

4. Not really, they are slowly fixing some bugged stats on armors. Also it is just bugged in general on how things all add together right now.. the games a bug fest. But, it will improve.

ok thnx for the answers