I cannot answer your other questions, but tamping is part of the process. The QP under my pool was tamped very thoroughly but while the original install was fine, the liner failed after 10 days. The warranty guys who re-installed the liner, didn't know how to handle sand and kept whining that I should put in a vermiculite base. Consequently, they didn't wet the sand, didn't use flats of plywood as "snow shoes" and I had wrinkles and ugly footprints for 12 seasons. I am now in rebuild mode so the liner and the walls are down.

We do have users that have tried the foam base. I'm going to use foam on the walls, but not the base. Perhaps someone can relate their experience with that.