I not want to spoil the party, but Immigration 2 is placed in a field next to Suvarbhumi airport. Furthermore I not see the Phu Nam Ron Check Point in Kanchanaburi, but they mark the inofficial Check Point at Sangklaburi, and in Chantaburi they mark only one border Check Point. Nevertheless it is nice to see the juristriction of the various Regional Offices.

yes. Bulgaria no problem.

yes, 30days extension if extend tourist visa

Yes and Yes.

you can change to a single non o inside Thailand, you need at least 15 days balance and have all documents ready. Including financial, however not every Immigration can do. Bangkok is OK.

no agencies that do Savannakhet and no agency in HCMC. However process is straight forward. Both places do not require "show money" in order to get the multiple Non O ( in case you referred to that visa).

@Carla *******************

the problem is not an easy airport in another country the problem is that you enter Thailand under visa exempt, the fourth in a row, if I would be IO I would grill you, you might consider applying for a tourist visa at an easy consulate and come in on that 60day Tourist Visa.

with a 90 day entry stamp you can get only 7 day extension, this is due the bilateral agreement.

the answer is 4. No, not possible. The other way around would be possible, your child can carry you on his Non B.

I think OP has all the option, if there is an issue with checking in than you better get a 60day tourist visa, if a Thai Consulate is nearby. Or just enter Thailand on 30 day visa exempt stamp and get a 30 day extension or do a side trip.