You booked a 1-week trip to apply for the DTV? Vietnam gives a 90-day tourist e-visa. I can only wonder why you this to yourself. The application takes 5-10 working days. If they ask for further documents, the clock resets to 5-10 working days. You should have planned accordingly.
That is not nonsense, it's nothing but your opinion. One of the categories of the DTV is about Thai culture. Thai food definitely falls under that. Might be problematic for extension but let's just live and let live.
I haven't even started yet, not thinking about what might be. I am assuming I'll continue Muay Thai as a means of keeping fit and learning some skills. If, I do not continue and it becomes an issue and they cancel my DTV, what I do not expect, I am sure I can then apply as freelancer. But I do not see a reason to worry about that today, it's my first day back in bkk and finding a nice condo has more priority in my head.
Try to contact the embassy and ask if they made a mistake as you applied for a different visa. They might correct the error or tell you that you are indeed not eligible in their eyes for the DTV. In which case you can always try again through a different country.
I booked 3 months for Muay Thai, just in case I would not be approved. Then I'd still be able to do it on a regular 60+30 day visa. If it raises questions, you can always answer you'd like to travel around and stay/train in different parts of the country. Which in my case will certainly be the case.