Tuition at international schools in Thailand are hundreds of thousands of baht per year, per child. Each one would require their own education visa, which means each one would have to attend school.
One child on an ED can allow one parent to qualify for a non-O based on foreign child attending school, and each parent would have to show 400k or 500k (I forget which one) in your Thai bank account in order to get your extensions.
Unless you have a few million baht laying around, which doesn't sound likely because you said you cannot qualify for DTV based on bank balances, I don't think you will qualify for ED either.
And retirement visa is not a good option if funds are limited either, as your mother will need to have 800,000 baht in a Thai bank account, basically forever as long as she remains in Thailand.
There are more nuanced responses to many points of your post, but at a macro level, moving a family that large to Thailand will be prohibitively expensive.
It's not about nearest office, it's about the office that serves the province you live in. Most immigration offices would have no problem, but very few require a long term visa.
The non-O visa has always stated for less than 90 days, because the visa gives you 90 days in Thailand. You can apply for an extension, but that has nothing to do with your visa application.
As long as you are currently on an extension based on retirement, there's no problem with applying for the next extension based on marriage instead. You just need to make sure that you meet the requirements for the new extension.
The people that have an issue are the ones that enter on or purchase a 90-day non-O visa based on retirement, and then decide they want to do the extension based on marriage instead (or vice-versa). They cannot do that because the first extension from a visa must be based on the same reason the visa was issued.
Normally, it will be issued the same day you apply for it.
Obviously every immigration office is different, so it's not a guarantee. You might get an under consideration if they want to do a home visit (rare for retirement but becoming more common).
I can't answer specifically about this form, but for marriage when they want a witness it is treated differently by office. Some just want a signature and a copy of the ID card and/or house book of the person who signed. Other offices want that person to actually go to the immigration office with you.
Seems like this is the same guy that posted just a few days ago about getting a bank account. But now is asking how to get a bank account? Seems like a big fat lie to me.