How much coverage do I actually need before it makes a difference?

Your plan is basially right. Full coverage on the outer skins, then targeted patches on the inner structure. The 'hit every square inch' crowd is mostly wrong for anything beyond the outer panels, you'd burn through double the material for marginal gains. The outer door skin is where you get the biggest payoff since it's the biggest resonant panel, so doing those fully makes sense. I'd add one thing: run a strip along the bottom edge of the inner frame where moisture and wiring live, that area rattles more than people expect. For the rear deck, focus on the center patch and anywhere the speakers mount, plus the edges if it's a large flat panel. The middle of a big unsupported panel is what rings, so a patch in the center plus coverage around the speaker cutouts covers most of it. On the inner door frame, prioritize the metal right around the speaker opening and the flange where it bolts up, that's what kills the most buzz. Small strips elsewhere on the frame are fine but don't stress about gaps. You're not wasting material by skipping the frame, you're spending it where it actually does something. On the cold garage thing, letting the rolls sit inside overnight is exactly what you should do. Cold mat is stiff and the backing fights you the whole time. A heat gun while you work helps too, just keep it moving and don't cook one spot till the adhesive gets wierd. Work one panel at a time, roll it down firmly with a roller or even the back of a sturdy glove, and make sure the metal is clean and dry first or it'll peel later. Wipe the panels down with some isopropyl or a degreaser before you stick anything. Doing it this way you won't be pulling panels twice.

I did my back doors a couple months ago and the part that almost got me was the plastic sheet inside the door. There's a black tar-like strip holding it on and I had to peel it back to get at the spots behind the speaker. Mine came off in one piece but it didn't want to stick down flat again after. Did yours come off easy or should I plan on buying some new tape for that part? Also don't cut the pieces too big ahead of time, I kept trimming them once the panel was alredy in my hands and it went way better doing it in place.

The adhesive strength is going to be more of an issue if you don't prep the metal. I've found that the metal itself needs to be scuffed up before you lay the mat down. Last week when I was doing a set of door skins, I spent about 15 minutes per panel using 240 grit sandpaper to rough up the whole surface. Also, when you are using the heat gun, only heat a 10-inch area at a time and keep it moving so you don't warp the edges.