DIYbanter - Advice draining a pool

We have a small fiberglass pool, 11x24 52 inches deep. The pool has some
serious stain problems from some metal plumbing in the filter system which
has now been removed. When it was closed for the winter and drained down
about a 3rd, I sprayed some muriatic acid on the stains which took them
right off. I've tried a zillion different other chemicals for below the
water line and it's still stained. I want to just drain it, acid wash it and
then refill it.

House and pool are on a sloped lot that is filled to be flat where the pool
is. Pool is enclosed and the cement deck the pool is installed in is 30x40
feet so no direct rain or moisture around the pool. I highly doubt that
there's any ground water under there at all and that's not my main concern.
I'm more concerned about the sides of the pool and side pressure when
drained and any downward pressure that the concrete deck is putting on the
pool. Pool company doesn't want me to drain it as it will void the warranty.
I think it's just a blanket policy because even when digging the hole it was
bone dry down to 6 feet.

Any advice about my dilemma?

Thanks...