Been drinking it in Rayong from a small village reservoir for six years with a three stage filter and a pre stage sediment filter and charcoal filter no problems
No, if you had a rentry permit with the O you would just be restamped until your extension date, with an OA visa you get stamped in for one more year if you return before the expiration date of the visa
If you want to get your possessions in first you either need to have a Thai wife who has lived in your country or have a job in Thailand, check the Thai website for Customs