Write a cover letter saying that you are staying at your friend place. If your friend got a rental contract then it'd be great if you can include that in the documentation. easy peasy
Well, you don't have to explain that to me but to the supervisor. All those thai visa pages are full of users who claimed to be interviewed in thai, some mentioned that they got questioned at the immigration at the airport as well. There was a huge crackdown on ED visas which has become a major red flag. If you can speak and read the basic then you don't have to worry, if you used the ED visa to live in thailand long term then that could lead u to troubles
I actually learnt more than that in 6 months, I could read some simple words and make a very simple conversation. 6 months of learning guarantee anything in between 1000 and 2000 words. The officers will probably check that you have actually attended those classes and they you've learnt something during these 6 months
6 month is more than enough to learn the basic, if you have attended the classes and done your stuff then you won't have issues. If you can't speak thai at all then it's gonna be a big red flag, ED visa has been to most abused visa during the past years, used for those who wanted to live (and work illegally) in thailand. If you can't speak thai then they have the clue to think that you just wanna stay there long time moving from ed to muay thai isn't going to help.