I'm not sure how you can apply pressure to the wall, but a sand bag on the ones you can would be a good idea.

As far as the sand washing out, we had something similar happen to us years ago. When we installed our first pool, we taped the hose to the top rail of the pool and let the hose run all night as it was initially filling. (We had been getting up every few hours through the night to take a look to make sure all was well.) When we got up the next morning, we found that the tape had come loose and the hose was now on the ground aiming right at the base of the pool. A footprint-sized amount of sand had washed out. How disappointing. That footprint remained the whole time until we replaced that liner. I just told my kids to keep away from the footprint and we never had any problems other than it wasn't pretty. Sounds like your washed out area is larger than ours was, but my guess is that it won't be a problem for you either. (I'd guess that mine was maybe 4"x8" or so.)

What a bummer. Sorry this happened, Anna.