Howdy - here are some answers for you:
1- ScreenOS and IDP - you do select what policies will use IDP. It is not a global setting.
2- NSM and IDP - when you buy and IDP device you get a license for five users for NSM. You do not need to buy NSM on top of IDP. You will of course have to get maintenance for that five user NSM license if you want NSM updates. (And of course you don't need that AV stuff......)
3- I had a customer who tested in 8200 prior to purchase and they were satisfied with the bypass feature.