Super glue.

I've been using Loctite 15 second ultra control super glue. The more liquid and slower cure time, the better.

Squirt it between the glass and loose pieces, let it spread, then press down with plastic wrap (the Walmart store brand will do). Leave it alone for 24 hours, then peel up the plastic wrap.

You can do it in small sections--you don't have to super glue everything all at once.

But, once you're done, seal the whole glass with triple thick.

It adheres the lifting paint back onto the glass, and fills any air pockets. Triple thick doesn't do that. It also allows you to to place pieces back onto the glass that have already fallen off and line them back up with some precision (instead of just blasting away with triple thick).

I've tested it several times on various backglasses. It works very well, and the results are much better than with just with triple thick alone. It works best on backglasses that are delaminating with pieces larger than about 1cm (like shown in the OP's photos), rather than flaking with small flecks of paint.

It does somewhat, but still typically laves air gaps that are visible when installed in the game.

First was as-is, second was super glue, then triple thick (not pictured), and finally installed in the game.

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If someone wants to take the time and do some touch ups later on a triple thicked backglass they can. You can take some novus 3 and knock down the triple thick.

With mylar, it's a bit trickier.