Murcielago
Apps are updated system-wide. You cannot have two different versions of the same app. This is not to be mixed up with two instances of the same app (the data held by the app).
If I deactivate "App installs and updates", presumably - as the name suggests - no more updates of (already installed) apps are carried out in the respective profile, right?
This means that profile doesn't have the ability to initiate installations / updates, NOT that the apps in that profile will be prevented from updating (as I think you're asking).
Murcielago But how exactly do "enabled" and "enabled for first party sources" behave in my setup?
"Enabled for first party sources", I assume, means Graphene's App client and the Play Store. Non-first party sources would be everything else. Hence "enabled" would include the "everything else".
Murcielago would "enable for first party sources?" be the best setting?
It would not need any setting as updating apps through your owner profile would update, regardless of the user profile having app installs/updates enabled or not.