Note that you are quoting RussT, but the statements you quote were made by me, not him, as he was quoting me in the statements you quoted.
Adopted theories should be consistent with the fact that we observe only one universe.
Here I disagree. Any number of universes, including an infinite number, is consistent with current observations, so long as the universes postulated or assumed, remain beyond our observational abilities. The fact that we observe one universe does not constrain us to demand of our theories that they include only one universe, but rather that they include only one observable universe.
Personally, I find multiple universes far easier to understand, and I see that as the direction in which cosmology seems to be inexorably moving. The point is that any theory which includes one and only one universe, has still to explain how & why this universe comes to be in its "fine tuned" state, which allows for us to exist & see it. We are delicate creatures, and it's easy to imagine some small variation on the constants of nature, such that there would be no chance of us, or anything like us, ever existing. It's the old anthropic principle problem.

But, suppose (just suppose) that there is a population of universes, each of which has its own set of fundamental constants, like unto ours. You can create a probability distribution for each of the fundamental constants, based on the frequency of occurance of each value of that constant, in the population of universes. Of course, we can't do that observationally, because we can't see any of the universes. But if we had a theory that explains how universes "bang" into existence, we could use that theory to produce the desired probability distribution. That will in turn explain why our universe is the way it is, without invoking any sense of 'fine tuning', because our universe simply becomes one of the elements in the probability distribution. All you need is a theory which does not make our universe improbable. String theory, for instance, seems to work quite well along those lines.
Well, theories are never true, they are only consistent. Consistency only means that the theory should not predict that we will see something, that in fact we do not see. A theory which "predicts" a zillion unobservable universes is certainly not inconsistent with the fact that we observe only one universe, because that's all we are supposed to observe, in the theory, anyway. Such a theory is consistent with observation.

Remember my quote from Hawking:

I take the positivist viewpoint that a physical theory is just a mathematical model and that it is meaningless to ask whether it corresponds to reality. All that one can ask is that its predictions should be in agreement with observations.

The part about "predictions in agreement with observation" is the simple statement of consistency. You are assuming that the fact of only one observable universe, means that there cannot be any other universe(s), but I consider that an unwarranted assumption. It certainly does not preclude the existence of an infinite number of unobservable universes. Consistency is maintained, theory & observation agree.

But there is one more point. It is not true that there is no evidence for multiple universes. Dark matter & dark energy are not observed, but are rather assumed to exist, as a consequence of observation. But how do we know that dark matter & dark energy are the most suitable interpretations? What if the other universes are not so "unobservable" after all? What if we have misinterpreted the observations, and the force we interpret as "dark matter" is really gravity leaking out of the other universes, and into ours? I can readily imagine a multi-universe theory, which includes such an effect, and therefore is not simply "consistent" with observation, but actually predicts the observed effects we call dark matter & dark energy, as consequences of the communication of information between universes.

I'm not here to make a case one way or the other, but I am here to make the case that observation should constrain theories, but not imaginations. And one should not be overly impressed by the concept of "truth", or even of "reality", as it applies to a scientific theory. The one and only constraint that should apply to science at all levels is consistency. Nothing else matters.

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The point of philosophy is to start with something so simple as not to seem worth stating, and to end with something so paradoxical that no one will believe it. -- Bertrand Russell