Hi Stuart;
Infinity is a tough concept, (probably one of the toughest .. as everyone says), but I think that the concept of 'nothing' has to be considered alongside of it.
According to the thinking, (and that’s what this is all about.. the theory follows on from the initial thought concepts and it isn't necessarily the 'truth'), the Big Bang didn't expand into anything. It expanded into nothing. The ‘thing’ it started from, (at the small end), was also infinite and dimensionless and therefore infinitely dense with energy. Infinite and dimensionless at the small end … expanding into infinite and dimensionless (and nothing), at the big end.
Eventually, (fairly quickly), it then brought into existence, the 'something' - all that we see.
In your example, I notice you're still seeking to describe the boundaries. Let ‘em go and the problem disappears. That’s the trick to all of this. Focussing on the boundaries just brings the problem back. Its only a thought exercise. Following ‘the rules’ and abandoning the boundaries, leads to an appreciation of the profound logic of what follows, and is very cool (IMHO).
It really doesn’t matter whether its true or not ..
Hope this helps.
Cheers & Regards.