Excellent points, Jake W. As good as Genus in my shower, at least
The symmetry or balance of swapping bodies is good, but there is a problem Lovecraft failed to address really: if a person from the present Earth is sucked back a million or so years to be a rugose tentacled cone slug for a season in a city surrounded by strange gardens with extinct plants, and the rugose cone slug takes over his body in 1919 or thereabouts, the future being going back into the past could give rise to "information" which violates cause and effect. Now hold on, I know what you're saying, and yes, your point was the conservation of energy so new universes don't have to appear ab nullio, with no apparent source for all that energy and mass, but I'm not defending cause and effect, I'm only saying Lovecraft is stuck in that paradigm even as he violates it, because he speaks of certain instances where the Elder Great Ones or whatever they're called faced imminent danger CAUSED by leaks of memory in the far-flung future. In fact every retrograde motion of a being back in time would present such leaks of impossible information that these new universes would have to branch continuously into being.
What I really loved about SOOT, which I'm referencing poorly above, is the part about how the Big Slugs bribe the carnal prisoners with library and travel privileges. NO, DON'T RESCIND MY LIBRARY PRIVELEGES!!!! THE COSMIC HORROR OF IT ALL!!!!
I take your point, OB, but perhaps time-displaced information avoids trouble by leading to its own cause. Many SF plots and cautionary tales of prophecy have used the 'can't avoid your fate' motif, from 12 Monkeys to Kung Fu Panda. Possessing the information may not necessarily lead to paradox, it may create a loop within time-space.