I can't answer most of your questions, but here are few things that may be worth knowing.
First, Ross does make O-36 track. It is not on their website, but if you ask for it, they will make it for you. I do find that the gauge is a little inconsistent, though. Sometimes I find a tight spot (the trains tell me where those are) and I need to pull out the pliers and work it over.
Second, not all O gauge track is gauged the same. Fastrack and Atlas are both very close to 1.25", but Fastrack has a rounded rail profile which makes it effectively wider at the top, compared to Atlas, which has a much more square profile. Because of that, my experience with Atlas track has been frustrating - I have items which will run fine on O-31 tubular, but will not navigate O-36 Atlas. Ross track and tubular track are both about 1.26". The extra hundredth doesn't sound like much, but it makes a difference. So, it is at least worth trying your long flatcar on O-31 Ross, to see how it behaves there. Gargraves is wider still, about 1.27" (at least the flex is, not sure about the sectional).
Third, have you tried the Hogwarts engine on O-31? I do not have one, but I have been told it does not run on less than O-36.
Fourth, the whole curve does not have to be the same radius. Many people create "easements" by using a wider radius track at the beginning and end of the curve, and a tighter radius in the middle. The trains look much more graceful entering and exiting the turns this way. If you can work O-42 turnouts into your plan, entering and exiting your O-36 curves, you can solve the turn radius problem and get a better-looking layout at the same time.
