Does Assault's might bonus last through the combat resolution step? For example, if Jinx Demolitionist (4 might) attacks with Assault 2, is she still at 6 might when conquer triggers resolve after winning combat?

Yes. Under the Unleashed rules update, Assault and Shield now give their might bonus until the combat showdown ends, not just through the combat damage step. So if Jinx Demolitionist attacks with Assault 2 (bringing her to 6 might), wins the combat, and conquers, she is still at 6 might when conquer triggers resolve. This means she qualifies as Mighty and can pay costs that require Mighty units (like Sunken Temple's conquer effect). Previously, Assault would have deactivated before conquer effects resolved.