Our church basement was painted with latex over at the very least 10 year old oil based paint. The team did very little prep, did not scrape old peeling paint so I am sure they didn't sand the old oil paint.
Questions:
There is nothing you can apply now to make the paint stick. The critical factor is the bond between the old paint and the new paint, and that is all underneath the new paint, so it is inaccessible. It's as simple as that.
It is not true that you can't overpaint oil (alkyd) with latex. You can, and it can work. What is true is that you can't overpaint a glossy surface with any expectation of success ...and... oil paints are pretty good at being glossy. The gloss is what ejects dirt, grease and other surface contaminants such as new paint.
I found a soup can I had used to paint with green marine LPU paint, which is very glossy. I had later used the soup can to paint black LPU. I had left about 1/8" of black in the bottom of the can. The black had "popped out" -- it refused to stick to the fully cured green. Needless to say, no other paint would stick to that LPU either.
If this wasn't done, it will be hit or miss what happens next. Expect it to fail if you bother it in the next few weeks, latex needs at least that long just to start really curing. After that, you'll have to see.
To make new paint stick to glossy old paint, you must remove the gloss. Proper sanding is more than is required. I am fond of a green Scotchlite pad, like you use to scrub pots and pans. Then wash it to remove oil, grease and dirt. Then a wipedown with the paint's solvent (e.g. water for latex).
New paint won't stick to new paint of a different chemical type, because it is still chemically active. Normal (1-part) paints take a year or so to cure. 2-part coatings cure like epoxy, and so they cure much faster.
No, latex cannot go over oil based without peeling and bubbling. You can give it a light sanding followed by a coat of gypsum (hope I spelled that right) then you can repaint
