You only need to have the latest and valid one as of the application date to apply for an extension. Only the latest is valid if you have multiple TM30 with check-out date open.
No issue at all to apply for tourism extension at Hua Hin office as long as you can show the "current address" (on TM30) in Hua Hin as of application date.
By the way, you entered on visa-exempt scheme, not visa-waiver if you can apply for 30 days tourism extension.
, that is "NOTIFICATION FROM FOR HOUSE-MASTER, OWNER OR THE POSSESSOR OF THE RESIDENCE WHERE ALIEN HASSTAYED", or Notification of residence for foreigners. Your landlord or hotel is supposed to submit it with TM.30 form for you and give you a receipt.
, you cannot go to e-Extention floor on your own to apply for an extension as walk-ins or online booking through immigration site. You have to apply for an extension in advance through the special website to use that service.
You can go as walk-in to either office in Chiang Mai to apply for a tourism extension. The entry point doesn't matter. You need to have a TM30 receipt with an address in Chiang Mai office serving area to apply in Chiang Mai.
Attached is typical requirements information for tourism extensions. At Bangkok, it's rather the hit-or-miss situation about TM30. You may or may not be asked. It's your choice to try without it first.