Congrats on the nature of the letter, and sticking to your guns. Please explain exactly how you convincingly approached your treatment plan in the letter as being better suited to the DTV.
all visa applications are online now, no embassy visit unless you are called in for an interview. You choose the particular embassy to apply online according to where you are physically located.
Initially a 6 month course will suffice. If you border bounce at 180 days you won't need to prove ongoing activity, but if you do an in country extension you will need to update and prove basically all the initial application criteria, plus some.
Regardless, the business surely is registered in some jurisdiction, go there and screenshot/link the registration, same with the companies website About page or mission statement page etc. Get creative and innovative.