I can't answer all your questions but I would say this. I have 8.5j wide 19" tyres at the front which are 235/35/19 in my base Giulia and there is plenty of room. My wheels are QF size and offset. I can go 245 if I wished. As long as you keep the rolling diameter of the 20" tyre the same as the 17" tyre, 225/50/17 which is what I had originally, I dont think you will have any issues with 8.5j assuming you keep the same offset as the QF. Others with more knowledge will be able to guide you better.