I would put trying to remove damaged lens coating towards the bottom of the list of possible repairs. Using a solvent to remove the coating would be preferable to abrading it off but to use a lens coating solvent would probably entail disassembly of the lens. Not a casual action. This lens is more complex than most interchangeable lens. There is the water sealing to be dealt with.
Since Nikon doesn't appear interested in doing a repair his options are somewhat limited. They would be.
Replacement of lens
Finding someone with sufficient skills and parts to repair the lens and especially the waterproofing
Removal of whatever the discolouration of the rear element is
Using the lens as is
Disassembly and selling as parts
Finding a garbage can.
Having, years ago, worked in a camera repair shop, I have seen others do this type of work. Always on a last hope basis. And that repair shop shut down this month. Retirement of owner.
I had an issue a couple of years ago with several Nikkor lenses having mould problems following a trip to the sweatiest place (I think) on the planet - Sabah rain forest. Foolishly I didn't put them in bags with silica gel. I had huge repair bills from Nikon and two lenses were written off including my beloved 60mm micro.
If Brasso might have been my saviour..... (sigh).... what might have been.......