I understood all that from your post. I’m just saying that if the distros end up being as inflexible as you’ve described, you may need to look for a way to get flexibility at a different level of the “stack.”
You can add and launch arbitrary non-Steam games from Steam, right? Can you use Steam to launch a script that moves around files in the background and relaunches Steam? And have a named launcher to “switch” to each user?
I don’t know anything about those distros, but if there isn’t a good way to do it here’s a shitty one: maintain a separate OS partition/installation for each person and have the “login screen” be the bootloader menu.
The official site says the DRM-free download includes both Windows and Linux (and Mac), just like the original game did.