Tuning newbie here.
For all mountain, some sites say keep the whole edge sharp, some say detune contact areas, other sites say only detune nose/tail PAST contact areas, etc. Which is it?
These are all essentially saying the same thing. Your effective edge essentially does end at the contact points. Keep it sharp to those points. Dull it slightly thereafter so there isn't an off chance of the tips catching.
At one point I stupidly took a couple passes with the wet pocket stone (some site said to do it) and I believe that I detuned my whole heel edge...
Yes you probably did. Now you need to resharpen it.
Anyway, I took a LOT of passes with the 90 deg Dakine edge tool, and despite how much material I scraped off, I was never able to make it as sharp as toe-side (using the finger nail test). Is this normal? I've ordered a better edge tool with multiple diamond stones, as afterwards I read that this is what I really want if I'm aiming for sharpness (vs changing angles w/ the file).
Two different routines and tools used for different reasons. You need a file to actually cut a fresh edge on the board. If you "polish"it once a week thereafter with the diamond stones it will help keep the edge sharper.
The other concern that I have is that I think that I took A LOT of material off of that side edge. I was cleaning up the dust on my work table and had to brush off quite a bit. The edge itself looks fine, but it made me wonder: "how much filing can an edge take over the lifetime of the board?"
You don't need to take a lot of edge off. But you need a good large file holder to keep the edge angle consistent. Just pull the tool "toward you" in short strokes. If the two edges don't feel same its probably the inability of the tiny pocket tool to keep a consistent angle.