1 scenario which is working and tested.
Used my Ace card in others GPay app, got cashback without any issues (cashback credited to card statement not GPay account).
Note: Here, I do not use GPay app, so using others app. So, at any time for me only 1 account active in GPay.
multiple active accounts in GPay didn't tried, so not sure about that.
Also, now I cancelled my Ace card, so not sure other person will get invite or apply offline (so far none).