I'm not familiar with mold allergy and its treatments, but if you believe there is a chance that this is what causing your drop in BP than I would try to treat the allergy first, because I prefer to treat the cause and not just the symptoms.

From my experience (and I don't know how is it to others) the effect of high fluids and salt intake is very limited without Florinef. From what you wrote it seems that increasing the salt intake won't be enough. But I might be wrong... this is only my experience. When I try to increase the salt intake (without florinef) the POTS gets better for a few days and then my body probably adjust to it and it stops helping.

Take into account that when taking florinef it is important to get ~3gr sodium (~7-8gr salt), otherwise it won’t be effective. I found it very difficult, since I don’t eat processed food, meat and dairy, and I couldn’t tolerate it, had GI problems, lost appetite (so it was even harder to increase the salt intake), stomach ache nausea, etc. And it caused my iron levels to drop so I was very tired and weak.
When I took florinef many years ago, before I started to keep on a healthy nutrition, I didn’t have a problem with the salt, and I continued this treatment for 2+ years.

Regarding Florinef vs Midodrine, I tried just the florinef, a friend who has POTS is taking Midodrine and it helped her considerably.
Have you done tilt with catecholamine (norepinephrine, epinephrine, dopamine) tests, vaslava etc. ?
My friend and I went to the same POTS doctor and according to these tests he recommended on the Florinef for me and the Midodrine for her (and in both cases he was right, although I couldn’t tolerate the high amounts of salts when I did get enough of it my POTS symptoms were great).

Anyway the good side of all of these treatments is that although it might take a while to adjust the dosage and find the right combination and timing, it is pretty fast to know if they are helpful.