35 000 000 ~ million people came to Thailand in 2019. The ones that get refused are the ones with a history that suggests they are most than a "tourist" if that's all your history is then you'll be fine, unless you've omitted some details.
You don't need a return flight to get a SETV or METV. But I would advise your husband to have secured a job before coming. The dive industry here is fairly saturated and unless he is managing one it would be tight for a family of 4 to survive on the wage.
My personal experience, the admins may not like this since it's not what the law says but this what happened to me when I left and reentered the country and hadn't cancelled my WP or extension.
my NON-B extension of stay (which I've had for over 4 years, Irish passport) was due to expire on March 31st. I left the country (flight from DMK) for Indonesia on March 25th, at this stage neither my WP or extension of stay had been cancelled. On leaving the country the IO indicated that my extension of stay was expiring on March 31st and asked if I would be getting a reentry permit or not. I told the IO that I wasn't that I was finished working and he stamped me out and sent me on my way no further questions asked.
I returned to Thailand (flight to DMK from Indonesia) on April 5th. The IO officer confirmed with me that I didn't have a re-entry permit and that my NON-B was expired. I told him that was correct and that I was no longer working. He stamped me in on a 30 day wavier again no questions asked.
It depends entirely on the officer that processes you. What things might trigger an officer to that? Things like your previous travel history, your appearance - do you look like the sort of person who will be selling hugs on Khao San to fund your travels, did the Chinese tourist before you wind him up, are you staying at an address outside of the usual tourist areas or it could be because he feels like doing his job
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