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Interesting question. My perspective is in context of physics honours about 20 years ago.
Your last question seems to be premised on a very deep and widely misunderstood issue, which I'd paraphrase as 'where is the universe'. Simply, if the universe is 'somewhere', then obviously all these other universes are 'somewhere else' and so you should be able to go to them, somehow.
I think this is related to how people often think of the 'geometric' view of General Relativity (which I seem to recall Einstein didn't like). This describes space-time as 'curved', begging the question of what is the straight thing that it is curved with respect to. This seems to imply again that the universe is 'somewhere', sitting in a flat background place, being curved against it.
All this is of course supported by our day-to-day intuition that all 'things' are 'somewhere'.
In fact (as far as we can tell) the universe is not anywhere. There is no outside or background or 'edge' you might eventually run into, or place you might stand and point to the universe. As to the geometric interpretation of GR, this is really only a useful way of visualising the equations to make them easier to understand and solve, and can be entirely done away with in favour of just solving the equations and deriving values for observable quantities - hey presto, no curved space-time and no need for a background.
So, applying this to 'multiverses', each different universe is also not anywhere, and the question of where they are in relation to each other becomes meaningless. Essentially, the concept of reality is practically restricted to inside one's own universe. Strangely, this seems to make meaningless the question of whether the universe itself is real, as it's not clear what you mean by real when talking about the universe.
There is one (very very speculative) counter-argument to all this that I've heard. That is that the reason quantum computing seems to violate 'normal' restrictions on calculation power is that it effectively performs some of its calculations in those other universes simultaneously with our own, implying a connection.
If this makes your head hurt, rest assured it makes mine hurt too.
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