Dee, I'm at a little of a loss to tell you how to word the modified contract, I'm not a lawyer and have no legal training - however, why should that stop me?
First though, let me comment on using the fire hydrant - It's a very viable source, the town charges you for the water used and the fire dept. charges for renting the hoses, etc. We always use water trucks to fill the pools enough to 'set' the liners, the cost is on the pool owner. DO NOT :!: let them direct the flow onto the floor of the pool, the pressure will cause wrinkles - rather, have them direct the water onto a wall in the deep end, after there's ~ 2 & 1/2' of water in the deep end, they can turn the hose to hit the water. If you have a MD, let the water flow until there is ~ 6" of water in the 'hopper' then kill the flow and they can install the MD faceplate and cut the liner out. (Honest to God, if these guys need to be told how to fill a pool, you chose the WRONG company!)
Back to pseudo- legalease :
Referring to the PB as 'scumbag' or 'jackas*' is probably not a good idea (satisfying as it may be

)
What I would do is write up what you want & expect from the job (like, a wrinkle free liner with no holes in it and no dips or lumps - larger than ~1/8" in the floor and NOTHING!! sharp or protruding under the liner and things like no leaks in the plumbing or from where they have to cut out the liner for suction ports and returns)
Then add something to the extent of; I expect the work to be good for another 6 months AFTER the pool is opened in the spring and has been in use for that time - (a legitimate concern, the pool will probably be closed right after the liner is dropped, with a cover on it, you can't see what's going on nor feel around for potential problems) {speaking of which, have the lines blown out of water BEFORE the new liner goes in, this is a perfect time to clear the lines and have them wait until spring to cut out the liner, if they don't cut it out now, there is no chance of water entering the lines and freezing}
Add something to the effect of "since they are obviously, as shown by the results of their first attempt to install your liner, not reliable in their ability to properly install THIS liner,, -- should they fail again you want $#### to procure a new liner and have someone who can do it correctly install the new liner"
I'd also add something stating that if they are to go out of business, you require 30 days notice (so you can sue them while they are still in business) and they are still fully responsible for fixing any problems with the new liner and everything that they were responsible for as per the original contract.
Don't forget to have them remove all offensive vowels
Type up the above as 'legal - sounding' as you can and run it by a lawyer for the polishing touches and to be sure that it's a document that would stand up in court! It may cost you some $ to get the lawyer to do this, but it's a consultation, so shouldn't be too much - then you also have the lawyer 'in your pocket' if things come down to a lawsuit.
That's my BEST advice for you - I'll be here for an 'expert's' appraisal for you or your lawyer if it comes down to it (though I hope it won't and that the new liner is everything the first one SHOULD have been)
Edit:-- I forgot to mention putting in a clause that would make the PB have to pay any legal fees (court costs and your lawyer fees) if you end up having to take him to court
