Update: I decided against spinning it because it wouldn't actually remove the polish - that only comes off with actually rubbing it off. It doesn't soak, rinse or scrub off in water and it's almost completely waterproof. The leather only saturated with water from the underside.
I needed something strong enough to penetrate the dubbin-type polish (as identified by the leather professionals on the leatherworker website) and they recommended I use turpentine.
I had seen washing powder be suggested and decided to start with that which we already have before purchasing turps (had already tried acetone, non-acetone nail polish remover, Saphir Renomat, fairy etc) and it was the best out of everything I've tried! I've been slathering powder mixed into a paste onto the bag, leaving it for a while, then rubbing with a cloth and it removes so much more. It saturated the cloth with thick black goo whereas any other chemical I tried barely greyed the cloths.
Unfortunately we were already running out of it so I'm completely out of washing powder now. When I can, I'm going to pick up a few packs of the cheapest washing powders I can find and get to work. I've barely made a dent but seeing the goo come off was so motivating! The process was easier on damp rather than soaked leather so I won't dunk when I repeat the process - I'll just lay the paste right on.
@pammbw just thought I'd tag you to let you know.