1-I use a WO 0.8 flatner with my WO M90. But only because it is designed for that telescope. Other flatners maybe OK but trial and error could be expensive. Try without and see if you need one.
2-LPR filters come in many flavours as you mentioned. It begs the question what kind of imaging you are doing? If visual you won't see much with only 80mm and an UHC. UHC is only for larger diameters visually. If CCD then is it mono or colour?
3-The orion st 80mm is good for all sorts of guiding.
As you have an EQ6Pro you can easily carry the weight. I think that Losmandy and WO offer a side saddle mounting option that might just be the ticket...............ENJOY YOUR IMAGING...........
