Some news reagrding the issue returning once you temporarily fixed it:
Unfortunately deactivating background data was not enough! 
Well, seems clear, blocking mobile data (in background) doesn’t prevent updates over regular WiFi 
.
So I guess in order to prevent Google Play Services to update to the buggy version again you either need to deactivate Play Store background data + turn off WiFi, …or outright deactivate Play Store altogether!
I updated my original post accordingly.