As long as your embassy writes the income letter it doesnt matter if your work money comes from abroad. I have my salary from Sweden and non o visa from within Thailand. Going for the 1 year extension this month now.
Well for marriage it is only 400k and not 800k or a salary at 40k/month instead of 65k/month. And with the marriage visa you are allowed to work if you get a work permit. Working is not allowed on a retirement visa.
Who said it was the only criteria? And yes if anyone wants to learn then can learn fluently within 1 year and they will also pass the primary 6th grade language test after 1 year of language studies. But only if they are serious in learning.