First thing is first, you'll have to get the children passports. That requires approval of both parents along with required signed forms and other things. Your girlfriend should be able to find the information about that in Thai language fairly easily.
If you plan on switching back to work permit in less than a year, don't waste the time and money to apply for the DTV now. Just work remotely as you are currently doing and switch once you get the BOI company setup.
The legal requirements to enter as a tourist do not change just because you have a visa. You can still be asked for proof of onward travel, and immigration can still ask for the same 3 proofs as someone entering as a visa exempt. Tourist is tourist.
Assume they booked the onward ticket to submit as part of the tourist visa application, and then tried to use the same one at the airport. Should have purchased a new one, or had the purchase of a new one on stand-by for check-in.
It doesn't matter how many days you have left because when you leave Thailand your entry stamp is invalid as of the moment you cross the passport gate.
There are 3 "retirement" visas that I can think of off the top of my head, and there are possibly more. Just because that's the first one you found, doesn't make it THE retirement visa.
They're about the most complicated extension that can be applied for. The local offices are not even allowed to approve them and they have to be sent to a higher level office for approval, hence the "under consideration" period for marriage extensions. They often try to push people over 50 onto retirement extensions once they hit that age.
It would help if you did the research to determine that the entire premise of this post is incorrect. E-visa is standardized across the world. The non-O is available in Spain, as it is available everywhere else.
I haven't previously heard of anyone needing a 4th flight ticket to show leaving Thailand. They've just needed the 3 tickets proving they need to "enter" Thailand multiple times.
There's not anything particularly special about such a case. You simply apply for extension based on marriage to Thai instead of work. You just need to get the requirements for an extension based on marriage so you make sure you can meet those requirements when you apply.
They may require you to formally "cancel" your work extension when it runs out, but they also might just let you apply for the new extension directly without doing so. That would be the question you would need clarification on from the immigration office. But the actual application is normal.
There is no DTV qualification for visiting family. It's completely unrelated to your application, and cannot help it. If it cannot help, the only thing it can do is hurt it.