If you buy a lotto ticket in Thailand on the black market (illegal) and you win, u will get paid (legit). The opposite would be a scam. You pay an agent or buy a ticket and u don't get paid or the agent runs with ur money.
It's called bribery. Illegal but not enforced. Agent, IO, and others down the line each take a cut. It's a common practice in Thailand in many fields. IO leaving is not a problem. Once u get the stamp you are legit. It's the same stamp as everyone else who doesn't use an agent. The IO put a real stamp, he just take a cut allowing you to go around the law. It's complicated to explain if u are not familiar with Thailand's way of doing things
The difference in price should be massive. If it's not then the risk is the exact same. If it's 5k baht ish and he just needs a babysitter then he has nothing to worry about.
I haven't try BKK entry since 2016. I'm not messing around with that airport. +, we don't know if the work permit he usually got was with an agent or not.
Non-O won't change anything in BKK airport, they are annoying af with cases like OP. He could land entry from Laos, very easy on visa exempt entry. I think if he was over 90 days they would have said so upon leaving, so he probably got lucky.
90 days+ = ban for 1 year. BUT... it's exactly 90 days so he might get lucky at the airport. BUT... he need to avoid Suvanabhumi airport, as they are ruthless with cases like your husband. I suggest that he fly to Phuket and you meet him there and you try to talk to officer yourself and explain the situation with the kids, etc. You might have to beg... but you have to do what you have to do...
Assist Thai Visa only have 467 FB followers, with 11 FB total reviews. I would strongly suggest you to NOT use an agent with such little following and reviews.
In comparison, Thai Visa Centre has 22k followers, and 1540 reviews.
In fact, if you have the 800k, you DO NOT need to use an agent at all. If you need help, use a lawyer that do not provide agency services.
In brief, it's bribery. The bribery in Phetchaburi is not recognized in BKK, and vice-versa. Went you pay an agent good money to help you, the agent pay IO and more down the chain of command. Only a fraction actually stay in the agent pocket.