Looks like most of the questions were answered.
Here is my humble contribution.
[deleted] How would you explain the following?
"My vpn also needed updating via play store despite neo store (fdroid client) says it's up to date."
F-Droid/Neo can claim up to date, usually, some apps get delayed on F-Droid compared to PlayStore.
aquila-enu I can't remember from which store i installed the app, but both stores always recognize your installed apps - both listed it under your installed apps - if they're available on both stores.
You can see this from the app info. E.g. - long press on an app, App info, scroll down to the "Store" section - App Details.
And all apps can see the list of other apps, that's why you got offered to update. Both stores see the app listed and it's version.
aquila-enu I do remember that for bitwarden, I got it from play store and when neo said it needed updating, it failed to update from neo. Not sure if that would apply the other way around.
Yes, it will apply because the apps were signed with different keys. And this applies also between profiles, too.