[Agrajag]

I still call myself a n00b as far as dom2 goes, but I can still offer my advice:
1) I'd say most of these aren't very good at all, because they require your mage to be in the midst of the enemy (I'm assuming you're talking about offensive PB and PBAE spells), and most mages are way too frail to withstand that, and even the more durable mages are still in risk. IMO they are useful only for thugs, and even then are hard to script properly for.
2) I think that means that the user has to pass an MR roll, not sure how hard to resist it is though. Just pretend you're trying to make sure your thug/SC is immune to the devestating MR spells.
4) Depending on available mages, more often than not I just use those spells as "filler" for when I haven't researched better spells (like AE spells), or when there's a high risk of fatigue deaths (since these spells often don't require lots of fatigue).
5) You can get Poor Amphibian or Amphibian summons in the water pretty early. Many of the undead summons are Poor Amphibians. And once you conquer an underwater province you can just get the ball rolling by adding in indy Amphibians/Aquatics. Atleast that's how I'd go about entering the water early.