I had the same problem with my male ambilobe. Tried showers and it worked for a while but he would close his eye again after a couple of days. I then got some silkworms and after a week of feeding those he opened his eye and stopped scratching it against branches. Not sure why this worked but 2 weeks after running out of silkworms, he went back to scratching. I can only speculate that maybe the silkies were better for hydration but I really don't know. My vet could not really find anything physically wrong either. I got the same result you did from the vet visit so I have been feeding silkworms as a supplement ever since and his eye seems better. That was 8 months ago and he has been doing fine.
PS. I have the tube flourecent from zoomed. Reptisun UVB 10.0 middle of the top of his cage.