Beautiful lot. I tested your lot and am pretty sure the problem is with the terrain level where the pools are. Something was just a little off with the terrain elevation at that location. It appears to be just a little low and slopes down slightly from the floor level of the 2nd level. I adjusted the terrain and replaced the pools and got it so that a sim could get in. Once they could get in, they could also sit on the edge. I also made the pools the same height of the 1st level and had no problems with changing the wall heights or adding/changing walls in the house. It took quite a bit to get it to work so it might just be easier to flatten everything and start over. The way I got it to work after flattening the lot was that I created stand alone rooms for the pool areas and then brought the terrain up to their level then put the pools in with the water tool. So it can be made to work but it was time consuming as it wiped out pretty much everything in that area of the lot when I flattened the lot.
Hope this helps.
I managed my sims to use the pool at last 🙂 Thanks to @CGrant56 I got inspired by the build a room idea. However, I didn't flatten or change the terrain level. I simply created a room around the pool areas with the wall tool and then bulldozed the pools. I build the pools once again and deleted the walls. Now my sims can swim 🙂
I still can't change or add walls in the house, but I don't mind. I really liked this house and it is ok not to change it inside 🙂
I think we can call this a workaround since we are still hoping for a solution from EA. The pool and wall tools were perfectly usable before the update.
