I think you may have a bigger problem than you realize. It looks like you had a leak from the return at one time. Since your SWG is right there, the water at the return is very corrosive, as evidenced by the severe rust on the upright as well as the bottom rail and pool wall. I have a feeling if you peel back the liner you will see a line of rust from the return on down, and it is worse at the bottom because the highly corrosive water pooled there. Water constantly seeks level, therefore it constantly pushes out in all directions. Eventually this constant pressure will cause a failure and the pool wall will rip along the rust line like a sheet of paper. If you try to slip something behind the wall while the pool is full it is going to make the hole larger. It already looks to be 2-3 inches across.
My situation started with rust spots bubbling up here and there on one side of the pool wall. As time passed the spots became larger and new spots were popping up all the time. After a couple of years we noticed there was a spot that had rusted all the way through, about the size of a golf ball. It was the end of pool season so I drained the pool down intending to take care of the problem over the winter. I never did and rain partially refilled the pool, above the now softball sized hole. When Harvey came I had put a submersible pump in to drain it back down. It was like trying to empty it with a straw, there was so much rain. By the time the majority of the rain had stopped my pool had gone from 12" full to 2/3s full. The next night the pool ripped at the rusted out spot and that wall snapped open like a switchblade. The force of it broke one of the top rails on the fence as it slammed into it.
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