Sorry you're having this issue! With a little care the collar can be removed without damaging the panels - I've done it a few times. There is no doubt that you got a substandard install. Your estimate of the cost to fix it is probably a little low and they are offering you ~10% of that to 'deal with' their mistake. Squaring a liner pool is a little tedious, but not difficult because once you establish one wall, it's pretty easy to get the rest of the pool 'squared' up with it.

I've built over 500 IG liner pools and am willing to discuss this with you more. I HOPE you get decent resolution on this!