If your landlord has already filed a TM30 and you are returning to the same residence you should only need to get the TM30 receipt updated at immigration. Well at least that is in Chiang Mai. If you are going to a new address your new landlord will need to file a new TM30.
part of the conditions of visa exempt entry is proof of onward travel at the end of the 30 days if not before. If you can't prove this they can deny you boarding. I would be getting either a tourist visa or a cheap flight out.
In Chiang Mai you take your passport with TM30 receipt and TM6 and they date stamp the receipt and update arrival card no. In their system. Jomtien may be different.
Alvaro as mentioned above several times. Tm30 return if address is the same, take receipt, tm6 and I take passport to counter 1, get a number and they will stamp receipt with date. Nothing more, no forms, no payment
Their Immigration and have every right to refuse you entry. To be honest if I was one and saw a foreigner entering with multiple back to back exempt and extended stamps I would ask questions also.
7 day grace period does not mean you don't report because you are leaving in that period. It allows you and extra 7 days to report if need be. You are still required to report your 90 day stay in your situation.