Sorry for coming in late on this one. Only had time to skim the 2 pages, so if I've overlooked too much please ignore me.
First off, if it's truly sand, it's not coming from your pool finish. There are no plaster type finishes that use sand. I could be wrong because I'm not familiar with them all, but that's my educated response.

A few years ago we replastered a pool and the guy started seeing sand in the bottom. He accused us of messing up his sand filter. The equipment pad was 25' away and we'd never touched it. After a lot of arguing & investigating, we found out he was using some bargain basement chlorine tabs that had sand filler in them. Chemicals desolved in the water, leaving behind sand. Just a thought.

Another scenario is if you have sandy soil and a leak in your return lines, it could draw in sand and deposit in your pool. I think that's very unlikely.

Third idea, which I've asked of others in this situation before, is if you have kids and a sandbox.