Closest thing I can see meeting most of your requirements may be a Pacific Cycles Reach SL, but I'm not sure it would meet UCI regs.
Seems to me the solution would be to keep your racer which I assume you still have, in a partially disassembled state in a closet, and use a folder to train and lug up and down the stairs daily.
For a truly lightweight folder or mini velo, you'd have better lucky building up from a frame set. An option worth considering:
Speedone mini velo and folding bike frame sets