B The distance to the nearest identical Hubble volume in light years?

[PeterDonis]

How can we say with any certainty that the universe has a radius at least $10^{10^{115}}$ times larger than that of the observable universe?

We can't, of course. This is one of those cases where it's not even clear how to estimate a probability. At our current state of knowledge, we can only treat Tegmark's statement as a consequence of certain assumptions whose probability we can't estimate. (The other key assumption, in addition to the one you state here, is that our intuitive picture, from what knowledge of quantum field theory and quantum gravity we have, that there are only a finite number of possible states of a finite Hubble volume, is correct.)