I was hoping I misread that whole thing about leaving the little one alone! CPS would jump all over that, and it would have absolutely nothing to do with being a homeschool family. It would have to do with gross negligence. I'd also be plugging up any hiding places if the child can hide so well he can't be found. Ugh- chills again.
I assume there is someone at home to watch him, since it sounds like he will be staying home every time the older one has swim lessons.
I also think there's probably someone else at home watching the 5 yr old. I wouldn't worry to much about him not being able to swim. It's a natural consequence of his behavior, not much you can do about it. I think what I would do is wait until your older son's swim lessons are over and then get something for them to play in the water with at home. I know it's not the same as swimming but honestly my boys seem to have as much fun with a water hose and a piece of PVC pipe as they do in the pool. A sprinkler or slip and slide, anything that they can play in a get wet and you can sit in the shade with your feet up and rest, plus it's much cheaper than going to the pool.
Another thought- I woudn't have pulled the 5 yr old from the lessons. I'd have made him go Tuesday, and at least the rest of this week. If he was upset over the ladder incident, I'd have explained the instructor misspoke when he let him go up, but it is their rule he has to be so tall before he can use the ladder.He must adhere to their rules, case closed. It might be a hassle, but by him wanting to go swim, too,that next day, he didn't really want to quit. He was just yanking your chain. Jeannie has pulled this, too, before, and when she has returned the next day and kept going, everything worked out.