I'm going to need a decent amount of time to learn all of these components, because I never use this OS, Tox, nor virtualenv :) I'll try and get to it, but don't hold your breath. Luckily, Modules/getpath.py is much easier to follow and modify than the old systems.

If there's a pyvenv.cfg involved, base_executable should be calculated based on the "home" key in it. Previously, I don't think we calculated it at all on Linux - it was just sys.executable before site.py changes anything.

On Windows, it was always intended to be "the executable that new venvs should be created with" so that venvs created from venvs would be based off the same install, rather than trying to chain. I have no idea what the correct path for that is in this context, so could do with some help.