Where is your time gap to get past ACH clawbacks?
However, we also think that the experience of instant transfers is valuable, so on a risk-based approach we allow instant transfers using ACH for certain customers and certain transaction sizes.
You have a fundamental misunderstanding how ACHs work. There's no such thing as an ACH that is "completed". At +180 days the state is "It is probably going to be fine". At +365 days the state is "It is probably most likely going to be fine" etc.
Source: Done consulting for execs at medium sized banks.
They don't need to classify an ACH as "completed" or "not completed", they only need to place a confidence interval on its completion state, which becomes easier as they process more transfers.
They ignore financial regulations to the level of Uber ignoring taxi regulations. The difference between the taxi commission or whatever its name is in San Fran and US Department of Treasury is that the later relies on people with guns and can block customer's ability to do any financial transactions.
The reason why in the standard custodial scheme ACH transactions are fine ( but still are frowned upon ) is because the appreciating asset can be reclaimed from a custodian via courts.