1- Can you sneak attack with any weapons?
2- If you take improved power attack, can you still use the regular power attack?
3-If you let those priest of Helm bless you, will you get the plague? If so, what happen?

1. Yes you can! Fists, daggers, maces, halberds, it makes no difference.
2. Again, yes. And the same is true for the (SoU) expertise feats. It is useful for when you want that extra damage, without making it too hard to hit your foes. Improved power attack, on the other hand, will certainly come in handy when there is a lock or door in need of bashing.
3. Yes, you'll get the plague, but no, I don't think it's scripted. It is probably assumed that it takes a while for the plague to affect the body, or at least the body of a true adventurer such as yourself. Just long enough to find a cure...

Thanks!