Phase 2 fails. Ended up extending the bottom supports and built a new top with slides so the seat can be adjusted.
This take introduces a few new issues. First. When the legs were parallel, the distance between the radii of the legs was the same, so all 3 lower supports could be even. When I flared out the legs, I did not account for the fact that the radii are now converging meaning that at least the back support had to be smaller. Second issue, as you can probably tell from my drawing is the slide slots mean there was not enough support. Between trying to make the bottom supports fit and slotting the new top, I ended up breaking a couple of pieces, so back to the drawing board.