Q4: I believe they did that to avoid having people quicksave/load abuse. The key to Haliwell's room, I'll give you that, but the others, those are things that are supposed to be "perfectly hidden", "no metagaming for you, Player." kinds of objects.
Q5: I do remember there being a few areas where you need some smidge of platforming skills to traverse. One of those areas is a part of the main quest, but I really wouldn't call it "a test of dexterity". There's one that's particularly difficult(time limit, small margin of error), but it's not important to the end game, so take it as you will.