that's a bit like saying the METV is a 6 month visa. While the visa is valid for 5 years it's not a 5 year visa, it's a visa that gets you 180 days each entry, just like the METV gets you 60 days each entry.
that's 4 out of 60 odd DTAs, I'm not going to check any of them specifically but I know many of them only give a pass to civil service pensions. Even the state pension is taxable under the UK DTA. Sweeping statements are the biggest issue, everyone needs to check their own situation
Paying tax is nothing to do with whether you have a kid attending school or not. The starting point is spending 180 days or more inside Thailand which makes you a tax resident, visa is immaterial. Then you have to look at whether you bring assessable income into Thailand in that year and if so you need to file a tax return if it amounts to more than 120k baht. If you pay tax elsewhere and that country has a DTA with Thailand you can claim a credit on the assessable income you bring in for tax you have already paid on that income
The Non-B makes sense as that is for working inside Thailand. I can't see other visas being tied to immigration as there are just too many variables for immigration to get their heads round.