5 gallons of 12.5% chlorinating liquid in 25,000 gallons would increase FC by 25 ppm so you'd be at 45 ppm if you started with 20 ppm. It will drop so it's quite possible you are at where you estimate.

With the 120 ppm CYA in the water, even 45 ppm would be equivalent to around 0.6 ppm FC with no CYA at 7.5 pH so isn't so terrible. On the other hand, the pH probably went up to around 8.7 so would further reduce the active chlorine level somewhat (though not that much).

I agree with Janet that whatever damage was done to your liner (if any) was probably when they poured the chlorine in initially, especially if they didn't do so slowly over a return flow and mix it thoroughly. As the water temps cool, the chlorine will drop much more slowly. As long as it stays above around 9 ppm, you should be fine as far as algae prevention is concerned. If you open in spring before the water warms up, I'll bet the water is clear upon opening.

Richard