Isn't that THB 750 comes from your monthly SSO contribution to the funds? Another THB 750 comes from your school with a total of THB 1,500 monthly. Just to be clear.
Its valid to drive internationally but once its expire, you have to go back to home country and renew another one. So its best to get a local driving license in Thailand and its easy.
AFAIK: No. But make a copy for them and bring the original with you on the day of extension. Ensure the monthly income is stated clearly in your currency 💴 on the affidavit.