You don't trade cash for the creation unit, you trade one creation unit of shares in the underlying securities. If 1 share of AMSETF is claimed to be equal to 0.5 shares in AAPL and 0.5 shares in MS, you hand 50k shares of AAPL and 50k shares of MS to the managers of AMSETF. They create 50k shares of AMSETF and hand them to you.

No cash changes hands (unless cash is one of the underlying assets of the ETF), so the ETF is never sitting on cash with the obligation to purchase shares.

In general, it's very unusual to hand cash to an ETF in return for shares (unless cash is one of the underlying assets). I'm not even sure it is legal to do so.