1. I think Weather the Storm is too strong of a talent, mostly because it provides "all resist" rather than picking an element to boost. I think Weather the Storm should be removed from the game. I do not think the concept of "I have over 100 resistance so I can heal" should be removed from the game. Having over 100 resistance and healing is pretty darn cool, providing the game doesn't set you up for a relatively easy "80+ in every single element."
2. Actually, it's not hard to imagine a ranged attacker making attacks from cover and remaining functionally invisible. Even with a melee character, it's not unbelievable so long as the kills are "silent." The problem has more to do with Sneak 5 reducing the AP cost to a ridiculously low 1, while Sneak is useless in combat with an AP cost of 4 or more. The concept of an ability reducing AP cost by 1 per point is not one which is possible to balance. It's the AP mechanics of Sneak which should change, not something which makes the "sneaky rogue" character type unplayable.
3, 4, 5, and 6. Agree.
7. I think Leech will need some added prerequisites in order to be a cool talent. If we keep it at "no prerequisite," we'd have to hit it so hard with a nerf bat to keep it balanced that it wouldn't be interesting anymore for people to take. Whether or not your version of it hit it hard enough with the nerf bat or not isn't really the point.
8. Yep.
9. Pretty much impossible to implement. It's a single-player game so we should be assuming players are cheating as much as they feel like. If you want to do this, just enforce the rule upon yourself.