How bad is the mylar on your playfield and does it have to be removed for the sake of game play or because you want to? For me personally I am happy to put up with old mylar if it isnt affecting game play and means the playfield artwork will remain intact.
I have heard of guys leaving the playfield in the sun on a very hot day to heat up the mylar. I have used the heat gun method but that was on a clear coated playfield and it worked well.
If you do remove the mylar just ensure you do it very very slowly.
Spot on - if you don't want to risk the paint, and the game plays well, best left alone. There are techniques to polish the mylar to a nice shine that will allow if to look good and play faster.
For the few games I have done removal, I found freeze spray
very ineffective. A heat gun works quite well, but you have to be prepared to go the next step either way - removal of the glue. The heat softens the glue, and allows the plastic to come away - heat and pull very slowly. I pull sort of "forward" so the lifting is minimal, allowing the pressure to slowly separate the two, rather than straight up.
Getting rid of the glue can be done with "Goo Gone" (Bunnings) but it takes a lot of effort. Some use flour and rub around to collect the glue, I tried that and it worked well too. I know someone posted a thread about that and they might be able to post a link as I could not find it.
