You can only extend once per entry, next time you will have to leave Thailand and re enter, the immigration officers themselves advice to leave and re enter to get fresh stamp
Morata qr transactions only works with qr codes registered as businesses doesn't work with qr codes of private vendors which majority of the small vendors use
May I know which passport do you have, if it's a South Asian passport, the immigration officers at the border might make problems when you do a border bounce, better to take a flight and re-enter from Airport immigration. Flights from Thailand are quite affordable to neighbouring countries.
you can purchase it online but I would recommend you to use a broker, they don't charge you any commission and will assist you with everything even in emergency situations