Conversion from concrete to liner is most certainly possible, I've even done it 5 times. However, the key to the process is a stable (!) shell with which to work. In your case, solving the sinking deep end must be the first priority. Whatever way you choose to re-do your pool, if the hopper keeps dropping out on you, you'll always have a problem with the pool (and if the bottom drops out, the liner's warranty is void).
I don't want to go into too much detail until I know more specifics on what is being planned (3 of the 5 have had some form of 'nightmare' involved in the conversion).
Wall foam is nice behind the walls of a liner pool but, there are reasons it's called 'wall' foam - again, more on that if you need it. But you will want to pad the liner from the ~rough concrete, however there are pitfalls with doing it.
I'd love to be able to give you a quick and comprehensive answer, but there are too many variables and I probably couldn't cover everything in one 3 page post, so for now - figure out what you can do about the sinking deep end, I'll keep my eye on this thread ( I will probably '"QUOTE" and respond' to a few of the suggestions already posted when I have the time over the next few days)
Good luck with your project! - Keep me (us) informed and I'll pipe in with advice and thoughts or suggestions