Interline baggage agreements have nothing to do with alliances. I am fairly certain that DL has an interline agreement with QF and presuming that you are on one ticket SAN-MEL, will check your bag through to MEL at SAN.

Your allowance on that ticket under US law is the allowance for the marketing carrier of the first segment. Thus, if DL markets the first segment, that allowance applies to the entire ticket.

None of this prevents QF from being picky and checking the weight of interlined bags at LAX (some carriers do this). You may find a nasty add/collect surprise at LAX. The same would apply to flying AA into LAX on a separate ticket.