1)I totally feel you.
I believe from form and function a R51 Pathfinder is the equal (and in some ways superior of) a mid trim late model 4Runner.
Albeit its TONS less money and so is well worth preserving.
I also intend to keep my R51 forever and a day.
IMO it is a future classic just like a late 90s 4runner or a mid 2000s 350Z.
2)The reason I said to wire brush and THEN to POR 15 is because the POR 15 already has rust inhibitor in it and I dont know how/if it would properly adhere to a bare metal that is slathered in inhibitor already.
I suppose you could use those "melts away rust to create a paintable surface" products after you sanded it.
Then clean those areas thoroughly and then add the POR15.
Even though.. the raison-de-etre of POR15 is to be able to skip that.
Dont underestimate how long this will take you.
Do you have access to a lift?
Or really good and tall ramps?
As for putting the FluidFilm on top of it. No need once POR15 is on there.
3)As for the rocker panel yes this should work.
Probably a better plan that just slathering it in Fluid Film.
4)There are huge areas inside the boxed frame you will never get to w/ a wire brush+ paintbrush though. So no realistic POR15 there.
For this reason, I would still recommend spraying lots of FluidFilm through all those air holes that reside on the side, top, and bottom of the boxed frame (I did).
It will likely collect just were the water ingress does and which means you will likely hit the rusty spots (if any) that way.
This addition is actually easy work compared to the rest of your elbow grease intensive to-do list.
If you are picky about your driveway lay out some newspaper in 2 layers or more beofre you do this.
The FF if you use a lot of it will drip onto the ground.
PS: Maybe Silverarrow will chime in because he has a lot of experience in this subject as well.