Welcome to the forum.
Back in 2006 I was having an issue with my inground stairs and wanted to see if I could build a dam to let me pump the water out on the stair side and work on them dry without draining the pool. The dam idea actually worked but never got to working on the stairs. However, the dam was a piece of 3/4 plywood probably 3x4 feet reinforced with 2x3s to handle the pressure. I formed a seal to butt against the stair edges using a real thick bead of silicone seal. To get the sealing edge flat I layed the plywood on a flat surface, on wax paper, and let the silicone cure for a couple days. The silicone won't stick to wax paper so when peeled off I had a good flat sealing edge.
Maybe you could build a 4 sided box and butt it against the pool side. You could then work on the return fitting from the top. You won't have to go deep so you shouldn't have the pressure issues I encountered.
I know I have some pictures of this thing somewhere so I'll see if I can remember how to post them.
Hope this gives you an idea.
Al