I have come to the conclusion that this is a poorly designed game. The combat sucks. it is way out of balance in some cases. I got in there with 4 characters in my party and never got the first bit of Offense as i had to move in close enopugh to do it and that was all the AP I had and even then, I looked up at the top and it had been stacked against me.
In general, you are not necessarily supposed
to get the chance to attack before the enemy does. You can choose to dump points into Speed & Perception & Leadership stats to increase your Initiative rating, if going first a lot is important to you.
The game/manual could have done a lot better job at explaining these things though.
As for wasting your AP to move to the enemy: you don't necessarily have to do that. And for the spiders in the windstorm, IMO chasing them is a very bad idea. Instead, you could, for examples: a) teleport them to you; b) cast spells on them from far away; c) shoot arrows/throw grenades at them from far away; d) summon creatures to attack them while your characters are far away; e) end your turn while you still have AP left, and wait for the enemies to come to you (granted the spiders often attack and then tunnel away right after, which is quite cheesy).
IIRC I think I beat the spiders by laying down tons of fire surfaces, so that they took damage every time they moved through any of them.
Having said that, I do agree with you that the environmental effects like the windstorms that make your AP recover super slowly no matter what (the Initiative/Speed/Perception/Leadership stuff I mentioned above doesn't fully apply to the particular spider section you mentioned, due to that dumb windstorm effect) and enemy buffs like Invulnerability on the Death Knights and other Void monsters are major aggravations which not well designed. IMO 99% of the traps/puzzles are equally as annoying too.
You are right that the windstorm spider section is stacked against you. That's supposed to be the gimmick of the spider land. I also did not enjoy that section.
As for your comment that the spiders keep coming: there are a lot of them, but they don't spawn infinitely. In a worst case scenario, you could kill one or two, then flee, heal up, and return. The ones you killed before won't be there again when you get back. So eventually you can beat them all by repeating that process.
For the Invulnerable Death Knights in the mine: to progress in the game, you are going to have to Sneak one character past them, and/or use Invisibility or something like that. That's an unavoidable 'puzzle.' I figured out where to go/what to do for sections like that by watching youtube videos that show players actually doing it (and the act of searching for and them watching them was another pain).
Even though you are right with most of the bad stuff you pointed out, the game also has a lot great aspects too.