Downwind abeam the CF the other day, I brought the CF to the top (DCT CF), and was 1700' high. Then selected extended centreline and it jumped to 3000 feet high. I guess this proves that it does assume you are on the extended centreline.
High speed, don't quite understand your last bit. If you select DCT CF (CF to top) then it will give you an accurate profile as the magenta line does a direct track to the CF. However, it does not update your current position as you move, so you'd have to keep bringing it to the top. Can't find anything about great cirlce tracks / discontinuities.
I might be able to give it a try if you describe how to do it. Do you mean create a discontinuity AFTER the CF?