What are the popular theories of Miri's earth & the one with the Comms & Yanks?
The original draft of "Omega Glory" mentioned that the planet was likely a human colony dating from the early space exploration years. Sadly, the timeline involved would be insanely off for the other points mentioned. (This really would have solidified the episode in a lot of ways if more hints had been thrown in as you went, rather than an absurdly bad final act.)
"Miri" is nearly impossible to explain, short of Magrathea. Even if you cite the Preservers (which is often used to explain TOS's worlds), you still don't get an
identical Earth out of it. Any theories start requiring 'three degrees of fandom' which generally equates to crap writing.
Here's a question -- are there alternate Vulcans? Or Romulus? Or ...?
Spock mentions that Vulcan
itself may be a seeded world, meaning that it may ALREADY be an 'alternate' in the sense of Trek's 'parallel developments'. Romulus certainly is, of course, since it's a colony of Vulcan. TNG shows a couple of more Vulcan colonies as well.
Straight up Miri-like clones? Never shown, but there's no more reason to assume that there isn't than there is reason to assume there is.