'mandy, I'm sorry for your husband's situation, and I know it's very rough on you, too.
Let's start with O2. I take it he's not using it at all right now, because he can't get tanks affordably. Not sure exactly what that means, but many people find that welding oxygen is easy to obtain and less expensive over the long run than medical O2. Can tell you more about that if you want. You say he has the correct mask: Do you mean the "Optimask" or "ClusterO2 Kit" that is made specifically for CH, or do you mean he has a standard non-rebreather mask? I'm going to say that 90-plus percent of people who use oxygen in the most effective way get excellent aborts -- flow rate of at least 25lpm; special mask; tanks (not a concentrator); effective breathing technique; and quickly drinking an energy shot at the start of the attack. If he wasn't doing it all that way, he should try again. We can walk you through it. (I'm going to guess that it might be hard to get him to try O2 again -- It's a perfectly reasonable but sometimes frustrating thing about people with CH: they just don't want to get their hopes up again and then be disappointed.)
I can't comment on that effect of the Trex injection, but do you/he know that the injections can be split so each one is a smaller dose? Most autoinjectors deliver 6mg; but virtually no one needs more than 2mg, or 3mg max, to stop an attack. A lot less expensive, too. Here's a file. https://clusterbusters.org/forums/topic/2446-extending-imitrex/
Note also that many people are getting sumatriptan in vials with syringes so they can administer their own doses.
His doctor was almost certainly wrong about his D level being too high. Strongly urge you to directly contact Batch (who developed the D3 regimen, and who will be happy to help you -- do not hesitate to reach out to him). Send him a message from here: https://clusterbusters.org/forums/profile/17422-batch/
Shrooms have to be taken every five days, usually at least three times and generally more for people with chronic CH. Triptans block the effectiveness of shrooms, so he can't use the Trex while busting with MM.