The root problem is poor adhesion between coats
Something has gone wrong with the latex application that is causing it not to stick to the oil paint.
A glossy surface under a microscope looks flat as a board, like the Utah salt flats. Stuff can't stick to that, this being the whole point of glossy paint. To overpaint it, you want the new paint to be microscopically jagged, have some "tooth" for the new paint to mold into and latch on. This doesn't mean heavy sanding, I do few swishes with a Scotchbrite green pad.
Usually, the problem is homeowners skip this step. And they get away with it most of the time, because most house paints are flat. They are already jaggy.
The other likely issue is that they painted the topcoat before the oil had time to cure up and become ready for recoat. The times in the instructions must be honored, even give them a little leeway.
These two layers will never bond. There is no magic juice you can put over top of the top layer: paint simply does not work that way.
Keep trying products
Latex is a relatively weak paint when it comes to solvents. The solvent for my 2-part Urethane paint makes a pretty good latex stripper, but it doesn't attack oil paints. I have also heard alcohol works well. There are also stronger strippers.
You also need to use strippers correctly. Follow the instructions and do everything they say, in particular, cover it with plastic sheet. Lots of people skip that step. When you find the right product and get technique dialed in, it should go pretty fast and lift the latex without damaging the oil too badly. Don't try to force it with elbow grease, expect the chemical to do 90% of the work.
All this to say: if it's dreadfully slow going, change methods.