1. It's difficult to say. Belgians, in general, tend to have lots of interesting flavors. Adding fruit may be too much. I would not recommend it. Then again, I've had excellent Belgian apricot wits.
2. If you want the flavor to come through really well, then add it to secondary. I would suggest this method. You can add it during flame-out if you want a subtle hint of it to come through. Just don't boil it or you will release haze causing tannins.
3. I would only add a little. Maybe 15-20oz.
4. I would keep it.
5. There's lots of debate on the "correct" way to do this. Different methods will yield different results. I've always heated the puree to 160. And all my wheat beers have had more than enough flavor. Read this: