2) I've been told that "immune" really means "a very high restistance". I don't know if it is good for anything, but I cannot recall any lich becoming insane.

3)It depends on if you are you playing dark or light.

Start by killing as many fire creatures as you can find. When you jump down in the first chasm, immediately run for the tunnel that leaves it, you can then stand at a safe distance and hit or shoot the behemots. It might be a good idea to leave them until later, you don't want to waste too much fire resistance on them. You need to kill them eventually because you need an area where you can cast invisible, or a light party might want to TP out.

When you jump down in the second chasm you usually find some ancient behemot blocking the way to the exit, although it might be possible to walk invisible to the exit, it ususally doesn't work, instead go away from it, hopefully there are not any ABs blocking the way, you will find another tunnel they won't be able to get into, or if you have a lot of spell points you can try casting sparks from the ledge. Since you don't get any experience in the breeding ground you might try another invisible after you have killed enough behemots and squeeze past the last of them.

4) Yes - except for the town portal part. If you go in visible there will not be any place where you can cast it(unless you have killed enough creatures behind a portal ), if you are invisible however there are usually a few places behind the east and west portals where you can recast invisible before it runs out. IOW use as many trips as you need to get the keys.