You are in a tough spot...I really appreciate all the help, I really do. Sucks to feel helpless but feels great to have community friends to help me. No wonder 95% quit before the first year is up. Frustrating.
If the formalin rids the tank of it completely, it's a possibility I would do it as a test as long as it wasn't crazy difficult. I'm guessing it won't work by taking out the inverts and putting them back in if they can reinfect the tank. I'd hate to lose them, but maybe it's the easiest and safest route if taking them out and back in risks reinfection. If the eDNA on the water works, that's an option too just to make sure it's what we think it is.
I suppose I can try to get new fish in a month or two when the tank recovers from the cyano from the dino phase and see how they do but it also sucks wasting hundreds of dollars just to see.
\Personally I would do the formalin to start. Or take the test, do the formalin while you wait... Then again after 10 days.. Or you could do the formalin for 10 days then wait for 2 weeks and then do the test to see if you ar enow clean. You just have to figure out the best way for you.