Wow. Welcome to CatForum and THANK YOU
for trying to make a difference and save this poor, abandoned kitty!
You really have tried a lot of great ideas. Unfortunately, they are all the same ones I would have suggested. I would *not* recommend sedating the cat, because as the sedation begins to take effect, they get a little freaked because they're feeling weird and their body doesn't work right. This can be dangerous if he retreats to an area you cannot reach and/or gets into a dangerous situation they cannot get themselves out of; traffic, in/under a car, other inaccessible areas or even missing a critical jump they usually are able to make with ease.
Can you try covering the cat trap with a large towel, so it looks more 'cave-like' and not like a 'cage'? You already know because of your experience that once you 'catch' him, secure him before trying to love and reassure him.
I once caught an entire litter of kittens in one large trap by rigging it to spring, not with the floor-trip-plate, but with me having to manually pull a string to allow the trap to snap closed. Doing it in this manner allowed me to wait until all of the kittens had entered the trap, instead of only trapping whoever first stepped on the trip-plate.
Good luck, and I wish I had more to offer. Please keep us updated, I would so love for this to have a successful and happy ending!