to make room for a 10sp cassette wheel i had to widen the rear triangle.
What is the chain slack situation for you after the fold? When I tried to mount a conventional rear derailleur on Brompton, I found that its tensioner struggled to pick up the chain slack, unlike the standard Brompton tensioner. My situation was additionally complicated though, compared to yours, by multiplying the front rings that added to the maximal slack. Have you had any issues in mounting the rear derailleur/mech?