Well, haha, at the risk of sounding like a Hubbard apologist, I will engage with the OPs questions on the terms on which he stated them.
IIRC, Hubbard never actually said in OT III that BODIES were transported to earth. If I'm wrong, then no doubt someone who can be bothered to look at the OT III materials will correct me, but what I recall is that people were in bodies on the other planets, were injected in the lung with, what was it, glycol? which allowed the thetan to be trapped in an ice cube and transported to Earth (he doesn't explain how it is possible for billions of ice cubes to be transported like that, but that's just a minor detail and would interrupt the flow of Ron-the-writer's story). So my understanding was that it was THETANS that were blown up around the mountains on Earth, not bodies.
In one lecture, Hubbard stated that he himself wasn't "R6'd" (the audience didn't seem sufficiently impressed, so he repeated it again emphatically, and you can almost feel him looking at the audience waiting for them to catch up with how impressive that makes him). And yet he still apparently had to penetrate the "wall of fire" (OTIII) himself, so presumably somehow thetans that were not caught up in the Xenu implants ended up in control of bodies that somehow are infested with BTs. Again, Hubbard doesn't explain how a newly-born body in the 20th century is born with R6-hypnotised BTs attached to it. You would have to embellish the plot to explain that. Perhaps it is the GE that brings the BTs with it into the body. I think Ron was probably busy sketching out the plot for another novel whilst writing up his ground-breaking research from OTIII, which is why he didn't have the energy to write up the whole plot. But don't worry, we have it all taped now, even the 36-day implant sequence of pictures has been obtained by Ron, though those details are not needed to run out OTIII, fortunately.
Fortunately, Alan worked out a solution for that, in his Route 66 procedure, which improves upon Hubbard's rendition of....oh, wait, I momentarily flipped into the valence of another frequenter of this board. Sorry. Who's Alan? Anyway, discussing OTIII can do that to a person: the charge on the incidents is so heavy that it can flip one out of valence, which, of course, is what an SP is, after all: out-of-valence in R6.