@Andy **********
1) You can only get 1 extension per year because there are only 365 days in a year. Getting an extension means you spent 180 days in Thailand and want another 180 days. With that math it would be impossible for you to apply for 2 extensions in a year. If you don't want to bother with extensions, just leave and return. You get 180 days EVERY TIME you return. That's why your visa is called multiple entry. There is no limit.
2) No one has ever reported a problem returning with a DTV at this point. You just handover your passport and printed DTV visa.
3) 90 day reporting is mandatory regardless of what you do. Each 90 consecutive days in Thailand you must file a TM47. If you leave before 90 days then you haven't spent 90 consecutive days in Thailand.