For the abandoned hall, if I have a unit there and play a flash and my opponent plays a crumbling sands, do I still get the +1 from the spell or because it was countered I don't

No — you don't get the +1. Crumbling Sands counters Flash, and a countered spell is not considered to have been played (Rule 425.1.b), so Abandoned Hall's "when a player plays a spell" trigger never fires for Flash (FAQ #10646).

One nuance: Crumbling Sands itself is a spell that does resolve, so it will trigger Abandoned Hall — but for the opponent who played it. They get to choose a unit they control there for +1, not your unit. Your Flash, being countered, gives you nothing.