update:
called US, they refused to switch the AA ffn for the US one.
they also claimed that the US ffn on its own (linked to US CC) would only cover the 1st segment, that I need to bring both CCs to check-in.
a) I assume she's making it up, and that 1st segment rules re domestic baggage fees. correct?
b) should I HUCA USair to attach my US ffn? can they do that?
(have not been able to access via Finnair,not by AA record locator nor by US one)