This assumes that each player only installs it once but games can be installed multiple times. I could even write a script that automatically reinstalls a game over and over again and thereby cause high cost for the developers.

Edit: Apparently Unity plans to detect and prevent fraudulent behavior. However, a user can still install a game multiple times for valid reasons. Something like Gamepass would also be very problematic for devs.

Yes, just tested it. I guess the app has API access but the paywall was not yet included in 6.0.2. It might get blocked at some point though.

Version 6.0.2 still works without paying. You can download it from F-Droid.

Infinity for Reddit actually still works.