Did you do cantelever edge? Bc the styafoam coping forms go into the liner track. So you can’t have a liner popped in there before the forms go in. Everybody around my area does liner and water absolutely last thing before filling up. I don’t see a way to fill with water while backfilling the walls…I installed the liner and filled the pool before they did the deck..... I think the as long as you backfill as you are filling with water it is ok....