Great post to read showing how you've well and truly taken control to get things progressing for you.
The D3 test isn't publically funded, so whilst your GP can do the form for you, you'll need to pay about $40(ish). The last time I did this was at Labtests, which are all over the Auckland area.
Feeling some trepidation when taking the last prednisione tablet isn't unusual, lots of people here report the same.
The full D3 info is at - Multimedia File Viewing and Clickable Links are available for Registered Members only!! You need to

or

.
The difference between the BOC mask and the one from here is like comparing food from McDonalds to a top restaurant. But hoping that all it does is to gather dust.
The MRI is to rule out other possible causes of the same symptoms. For probably 98+% of people who have reported back here, it shows nothing at all abnormal, but for some there has been a different cause of the symptoms including brain tumors, aneurysms and eye infections plus others. So it is good to rule things out (I agree about you getting some return from many years of taxes if you're not in Southern Cross or similar).
For info on alternative methods, have a look at clusterbusters.com. This isn't an area I have any direct experience in.
For your questions.
#1 - I kept my cylinders for a year or two after my last CH before returning them. If my CH returns then I know I can get them quick plus I've sumatriptan injections to carry me over. Return them when you are comfortable. Going to BoC direct you will pay about $4 per cylinder per month (prices from a few years ago), so not expensive to keep them. You don't get a refund for unused O2 on returning them.
#2 - People have reported from no symptoms at all to a significant reduction in CH but still having some. So somewhere between isn't unexpected. It is possible that your D3 levels are still building up to the level where you can go symptom free. There is more info in the D3 link I posted earlier.
With the 30C and 95%+ humidity I think anyone without air con is tired from disturbed sleep.
#3 If you went outside at noon in the weather we have wearing shorts you'd generate well over 10K iu in just a few minutes. Many of us have been taking 10K iu for years and other than higher D3 tests than most people (with healthy Ca levels), there seems to be no issues. But do talk with your GP.
#4 Some probiotics might also help, especially if you have antibiotics which kill off more than just your malicious bacteria. Good food helps you so much more than just in relation to headaches.