In theory it’s the house owner that needs to file the TM30. In practice it is you that would be fined for not having one. If you have no need for interaction with immigration then you probably wouldn’t need one.
If you need an extension then you probably will.
Office dependant on whether they’d actual need one from the first 24 hours of returning or just that actually have one filed.
No one yet knows the requirements to get an extension off a DTV. November 27th is the first date that someone could possibly try, but that would be office dependent.
It is known that leaving and returning will get you a new 180 day stamp.
You mention that you got a multiple entry extension but none of those stamps mention that. You probably just got the year extension, which is not multiple entry.
If so you need to buy a re-entry permit in order to keep the allowed to stay date alive. Single permit is 1,000 baht, multiple is 3,800 baht.
You can do this at any time prior to leaving. You can get a single entry permit at the airport before you fly. You perhaps won’t be able to get a multiple permit at the airport.
That’s going to be specific to the office you use. Some will require it, some don’t seem to care. If you specify what office others may be able to advise their policy.
The amount of documents that are needed to apply for a Non B in country is huge and most companies can’t provide all that’s needed, therefore it’s way easier to provide a few documents that are needed for a non B application outside of Thailand.
What documents that would be required is dependent on the embassy you’re going to use, and the documents need to addressed to them specifically. Check their website for requirements.