1. Im brewing my first pumpkin this weekend and have no experience with it, but I would assume there would be some sugars from the pumpkin.
2. Pumpkin can be very messy to use in your boil or in the fermenters. It can be very hard to rack off of, from what I have read. Stick with adding all of the pumpkin to your mash. If you think about pumpkin pie, the flavors are more from the spices anyways, not the pumpkin itself. I think the subtle pumpkin flavors, backed up by the spices is what most people are looking for. Try around 60oz of canned pumpkin, thats the number i have seen that works well. Also, make sure to use rice hulls in your mash if you put the pumpkin in there.
3. Again, no experience with brown sugar or molasses, but if you were to blend it right, im sure you could get great flavors from the spices and brown sugar. Also, one thing you could try is to bottle condition with brown sugar. That would probably give a nice brown sugar flavor without having to use a lot