I never used the nasal cpap mask - I always used the full face mask - did you try that?
You need to check with the doctor about low levels. Most people (in general) have low D except if you live in the south or west (sunshine states). You won't get the answers you need here unless you contact a surgeon because every insurance company and every surgeon is different. If you don't find anyone at Johns Hopkins, check with Dr. Elariny in Fairfax. He's an expert at the DS.
It would be best to get your D a lot higher now, while it's easier for you to absorb D. The prescription form may work now, and won't work after your DS because it's packaged in oil, and oil = fat, and we absorb very little fat. So post-op you will need the "dry" D that you purchase OTC.
Be advised that no one at Johns Hopkins does the DS. So, if you go there for a consult, you are wasting your time. There are DS surgeons in your part of the country, but not there.
Work on it while you are waiting but don't delay surgery for this issue. If you want a DS, you need to go to someone who does the DS. And beware there are a lot of look alike procedures out there that are not the real DS.
Think of this like car shopping. If you want a Toyota, you don't go to the Ford dealer. But that Ford dealer has a lot of cars he wants to sell you and he will try to make you settle for what he has to offer. Think critically and don't settle for something less than what you want!
I agree with the don't delay the surgery folks. Vitamin D won't matter to your surgeon.
As a practical matter you should do whatever you can pre-op to bring up all your vitamin levels.
It is just harder after the DS with the malabsorption in place. Better to be at a point where
you can maintain post-op. Not impossible post-op, I don't mean to suggest that, it just a lot
easier pre-op and gives you a great base going forward after the surgery.
Congrats on your decision and I hope the approval process goes smooth for you.