Spring start-up after 10 YEARS of zero maintenance

Who knows what lurks in the bottom of the pool. 25,000 gal Sylvan concrete / marblite pool built early 1980's and maintained by me for family fun until 2004. Since then, zero maintenance (divorce), cover badly torn, wildlife decomposing, etc. Now facing major start-up issues.
First question - should I try to treat the water in place or dump it ? Our goal is to get the pool to a swimmable, functioning condition (by July 1) and let the new buyers decide what major restoration they may want to do.
I remember how I would use a flocculant to drop the "stuff" to the pool floor - then just vacuum to waste. Very effective.
I'm thinking that the walls and floor will be discolored and the best treatment would be to clean the water and keep chlorine levels high to whiten it up, although whitening is not our goal - just functionality.

Is it best to NOT drain the pool and cause the walls to dry - does that create a tougher stain to deal with later ?
I'm pretty handy and have manipulated the large DE filter and pool pump to drain the pool, vacuum it at 1/2 full levels, etc.

Well water. We fill via delivered water. Plan to store 400 gallons for use in cleaning.