I had a similar situation early last year for similar reasons to you.

I did find a cheap solution which I know the "chemicals" guys are going to slam me for.
The cost was less than $30 (estimated fuel cost) for a temperature increase of 25 plus degrees in a few hours in a 6000 gallon pool.

I have multiple zones on my boiler.
I "tapped" into the laundry area zone with a regular garden hose on the drain/bleed hose bib.
I threw the hose on the ground next to the pool. I then turned up the thermostat in that area of the house to 90 degrees so that the circulator pump and demand for heat (Oil burner) was constantly running. I let it run for about 20 - 30 minutes, constantly checking with a bucket to see if the hot water coming out was clean. When it finally was, I threw it in the pool and waited 4 hours. I had to let “some” water from the lowest drain (THE COLD END) every hour or so.
By mid early afternoon, the pool had gone from 56 deg to 82 deg.

I tested the water with the Taylor kit (recommended by Ben and many others) the day before and the day after. There was little to no difference and the water seemed fine while swimming and was easily maintainable for the remainder of the season.
My pool cost less than $300 so I was not too concerned about what I might do to the liner. As it turned out, there was no apparent damage.

This is what I did. That doesn’t mean it is what you should do, just food for thought.