you need 2 certificates. One stating coverage for O-A visa and one for Covid coverage. The immigration officer checked both and stamped me in accordingly.
Nothing special to do. Your are now in Thailand. Create a new account and add this account to Google play and then you're able to switch between your accounts.
He can self file at Phuket immigration. Ask for the handout and don't forget the stamps for the power of attorney. There is a small food kiosk at immigrations which sells the stamp.