Haven't read the whole thread, but DS1 (now nearly 3) has/had two undescended testicles. He's had two op's to fix them. Hopefully, one side has worked (not in the scrotum, but further down), the other side is not really down, but may come down more with time.
Couple of points - I heard earlier the better, best before they're two for fertility/future health reasons. The first surgeon wanted to do both sides at the same time, but the anaesthetist wouldn't let him. It takes longer - so GA more risky, if they get an infection, it will commonly affect both sides, so you could lose both testicles, and is a lot more painful, really requiring a hospital stay. Also, no-one told us the potential failure rates initially, and I didn't ask. Apparently about 30% of testicles die on the first operation, 70% die if they have to have a second procedure on the same side. Sometimes due to infection, other times they just shrivel up and get reabsorbed. I found this a bit worrying, as the chances of my DS1 ending up with two functioning testicles are low Nothing you can do about it really, as you have to have the op I think, but poss worth checking you get an experienced surgeon?
DS1 came through both ops fine, and wasn't worried about the second one, even though the first one was only 6 months earlier. He was bouncing around the next day, with very little pain relief. I think it was worse for us than him.