Gandalf's armor and health are fine, that's not the problem. The problem is how noticeable he is, even in the middle of an army. Once you have GTW, he shines like a beacon, or in the case of your enemy, like a target. That is why he dies, because people focus him down first.
Shelobs web is currently suffering from the exact same bug that Corrupted Théoden used to have on his traitors ability, even though the cursor makes it seem like it will affect a single unit, it actually affects many units. The fix is simple, just give it a small radius, the same way you did to Theodens traitors ability. It would not at all affect her ability to kill heroes, which is her role, it would just prevent her from freezing entire armies for the slaughter.
Finally, I don't think that we should look at heroes being too weak or too strong on a general, faction wide basis, but rather should address them individually. For example, Saruman and Gandalf both die extremely easily due to them being so noticeable in the midst of an army, so maybe they should have more armor against arrows. I think that when you just do general nerfs or buffs on all heroes in the game, you end up with less balanced heroes then if you look at them individually, based on cost, how noticeable they are in your army, and finally how good their abilities are.