Sorry to hear of your recent problems with your adrenals, but glad that you have also had some good news.
Firstly, can I say you are lucky in that you have a GI who actually bothered to check your cortisol before tapering you off the steroids. The vast majority do not, including mine. And judging by some of the comments made in this thread, a lot of forum members have GI's who don't mention this extremely important subject.
I was tapered off pred countless times over the past 7 years, and got horribly sick each and every time. Not once did anyone mention adrenal function. I demanded a test, and like yours, I discovered it was dangerously low.
Firstly, being on pred can affect the results of a cortisol test. But, an endocrinologist will take this into account. I was diagnosed with Secondary Addisons Disease (as you have been), and was switched from prednisone to hydro cortisone. I have been on this for 5 months now, and a recent Day Curve test indicated my adrenal glands are starting to wake up. This would never have been possible on pred. Pred has a MUCH longer half-life than hydro-cortisone, and it stays in your body for a long time. Due to this, your adrenals do not get a chance to come out of hibernation. SO, it is vital to move off pred and onto hydro-cortisone. What dose are you on now? A 9 hour day-curve test will determine the correct dose for you. This is a standard test that all endocrinoloigists will perform on patients in your position. Too much HC and your adrenals will not wake up.
I was given an emergency hydro-cortisone injection to keep at home in the fridge in case of emergencies.
Your taper of 7.5mg and 0mg of pred, is what I used to do, for years. It is a complete waste of time. For people on short courses of pred, it is fine. I was on pred for 8 years - so it was definately not fine.
If you have dental work, etc, you need to take at least double, or 3x, the usual amount of HC on the day and for the following 3 days after any procedure. As you won't be able to produce the increased level of cortisol on your own.
But, secondary addisons disease is reversible, as you have already been told. I have already reduced my HC from 15mg in February to 10mg now. You should bear in mind that HC is much less powerful than prednisone. It is roughly one quarter the strength of pred.
You will be fine! It's just the taper will take a little longer, but it's worth it, if it means you avoid a flare-up. I only wish I knew all this years ago!
Hope this helps, Take care