At 900 a single error instantly loses the game and even then you also must move every 3 seconds or you lose on time...
How many mistakes should your average 900 blitz player make in a game? How fast should they be?
With respect that hasn't been my experience, yes theres blunders, but the vast majority are making best move nearly 90% of their total moves and their doing around 45 move games in under 3 min...

Untrue. I barely play blitz but I am 1600 in blitz and even people in that range make many mistakes.
I noticed a sharp skill increase in the 900 pool, people making 30 best moves in under 2 min seems to be normal now, so what 1800 blitz by that measure could beat a GM.. Something is off here.