1. Can you confirm that the basis adjustment is made on the "Form 1099-B Worksheet"?
yes
2. If on the "Form 1099-B Worksheet", which field to "override": "Gain/Loss", "Adjustment Amount", or "Adj(Gain/Loss)"?
the ETF should have sent you a sales summary for your sales. that is what you need to use to figure your proper tax basis in the ETF for each sale. The 909 might be correct but are any distributions showing in schedule L.
In addition, if you had multiple buys and sells during the year, you have to watch for wash sales which the broker won't catch because they don't know your actual tax basis. It could even indicate a wash sale when it isn't.
