You certainly need an active ongoing treatment while your in Thailand. And actually attending it. Thats the whole point. The medical one is not one to try and cheat with
Sure, then tell them you intend to travel within 3 months. Doesn't matter when you actually travel. The validity is still 5 years regardless. They wont cancel you visa if you don't enter once its issued.
Its very common to apply early with refundable tickets. You can without issue travel later. I didn't even provide (or asked for) tickets when I applied DTV.