When you look here how many applicants think they can keep legally using a visa even when they do not comply with the requirements anymore, that should be enough proof...
90 days reporting is not related to a type of visa.
You will be approved for DTV outside Thailand, so when you enter on DTV 90 days start counting for the first time of stay, meaning you have to report in person.
When you bounce after 180 days and get in again it will be again the first time and has to be done in person again.
Baht for DTV, if your soft power activity ends, you do not comply with the visa requirements anymore, so you should not use it anymore. You do not get any money back, even when you could use it only for a year.
I have no problem. It is quite clear London embassy made a mistake, for language education there is another visa in place. I think you can easily continue muay thai for five years if you want, but nobody forces you to do so, but you will not get a refund for the years you cannot use the visa anymore because the soft power activity ended.
Why are you glad Thai embassy in London made mistakes?
ED visa is for eductation, learning language is education.
Is it that important to you to save a few baht?
Besides that miost language courses do not last 5 years, so you might loose validity time of the visa of find another qualification ground when your language course finishes.