It’s not. Transit without collecting baggage and going through immigration requires one through ticket. In this instance the OP will officially enter Thailand and receive a 30 day stamp. Then collect baggage and check in for the flight to Cambodia and go through immigration again and get a departure stamp. If asked about the short time in the country the answer would be “I’m just changing planes”.
On return to Thailand she will get another 30 day visa exempt stamp which this time she will actually use while staying in the country.
So, the people who aren’t overstaying their visa, in fact people who’ve never even had a visa because they don’t need one in their own country have told you that overstaying a visa is ‘no problem’?
Legalisation processes in general are a complete farce. The uk government doesn’t check anything. I recently helped someone get his university degree legalised in the uk by the uk government and the Thai embassy. At no point did anyone check anything to see if it was real or not. If you read the small print on the declaration, all the FCO did was declare that the solicitor’s signature was genuine. He looked at the copies of the degree certificates and the transcripts for five seconds and signed them. No one during the entire long drawn out process did anything other than put a stamp on things. It cost hundreds of pounds in total. The Thai government departments both in the uk an in Thailand are quite happy with these stamps that prove that absolutely nothing was checked by anyone.
It doesn’t say how like no the monthly income needs to be shown for though. Is it transferred every month for a year? How would you do that without already having a yearly extension? Is it transferred for one month? I guess we’ll have to wait for someone to test it out at immigration.
The age of your son and the amount of grandchildren you have has no bearing whatsoever on how the immigration officials will react to the number of visa exempt entries you attempt.