they could maybe clear this matter with the staff that awaits them as they arrive at the Gate, where they normally would be forwarded

there is just the Welcome Mariachi Band at the gate . . all they have to do is walk to "Domestic Baggage Claim & Exit"

who would control them at the domestic exit at Suvaranbhumi? There are no cntrols

if they hold boarding passes and CIC stiker,s they are allowed to the international departure security screening and passport control

there are NO controls at domestic arrival

I hope you got enough time at hand between arrival from Krabi and departure to UK. A re-entry permit takes about 10-15 minutes each person

when you arrive at Suvarnabhumi, take the domestic exit. Go to the international departure hall and proceed to Departure Section 2

no, you still can do another entry by land this year (if this would be the second entry by land) and an unlimited (not really but some) by air. Why don't take time when you are in Singapore to get a Single Entry Tourist visa, it would give you 60 days plus 30 days by an extension for 1900 THB and can also be converted to a Non-Imm-O based on being over 50

you can apply for a "conversion" from an exempt entry to a 90-days Single Entry Non-Imm-O based on being over 50, on Immigration Office inside Thailand. In the last 30 days you can apply for the 1-year extension of Stay based on being over 50, if you meet all requirements (financial & TM30, documents etc) All on the assumption you are a Canadian with a Canadian passport who just happens to reside in Singapur. You could ask, however, the Thai Embassy in Singapore if you can apply for a Non-Imm-O with them, maybe your "resident" documentation will be accepted

for peace of mind, I would go to Immigration a few days before your visa expires, and extend with 30 days for 1900.- THB. Remember, maybe some day you might want to get the Elite Visa. One single day of recorded overstay, and you will never be able to get it