Have a GOOD look inside the bb shell.
If there is rust in there, check the bottoms of the chainstays and their sockets; check means probe *hard* with a dental pick or similar tool; significant rust here is frame death.
Then that seatpost will have to come out before you use an oxalic acid bath, unless you manage to keep the aluminium separated from the solution.
Lots of advice of various methods of getting one out if you search.
The problem with rust on good frames is that they are thin walled, so the pitting yours shows near the seat cluster would concern me.
Measure the seatpost diameter (average of several places in case it varies).
Subtract that from 28.6 and divide by 2 = wall thickness.
Less than 1.0 = good frame, rust bad.