Unless your Thai embassy will sell one of you a non-O based on retirement and sell the other one a trailing spouse non-O visa, you'll likely both need the money in the bank. Most embassies stopped selling the trailing spouse visa a while ago, and it's not available in Thailand.
Most reports indicate that immigration offices do not want to give this extension. They will throw up many barriers to prevent you from successfully applying for it, and suggest you do a border bounce by leaving Thailand to get a new stamp.
Here are the official requirements, but immigration offices are free to add their own requirements as they see fit.
Yes. But there's been many reports of airline staff asking to see your TDAC before allowing you to check-in to your flight. So you better have a plan B if that happens.
I don't care what other people tell you. It's not grey. This is how visas work. Embassies will not issue visas if you tell them your travel is not within 90 days. But once the visa is issued, it is valid until expiration date. Period. End of story. There is no system or mechanism to cancel a visa after that.
The official government website is your local Thai embassy. Each embassy sets their own requirements, so anything posted anywhere that is not the place processing your visa application is irrelevant.