As for the white powder, it seems to be on the leather (dated) gasket only. It may be salt(?) caused by a salty air, which may also explain the extremely fast rust.
I’ll follow your suggestion and remove it for sealing.
Do you know if accordion repairmen are aware of the need to operate at a humid reed?
(it’s a bad question because your repairman is not mine, but it’s a long shot for using your experience)