> But the value as it's calculated now seems to give a file that doesn't exist - how can that be correct?

Because we never actually use the executable referenced by the 'home' path in a pyvenv.cfg. It's only used as a starting point to locate a couple of entries in sys.path. So if the executable isn't there, it's no big deal.

It shouldn't even matter in the nested-creation case, because we still don't execute it then. It's just how we end up with the same value for 'home' in any venvs created within a venv.