Fentanyl dissolves well in iso but the binders don't. You filter out the I so from the binders and the fentanyl is dissolves in the iso. Evaporate it in a shallow container and you should be left with a microscopic amount of fine white powder.
91% will probably dissolve some binders. I would try to find some 99% or dry it out with some Epsom salt.
Use a small container to evaporate if you can and try to keep the amount of alcohol small. Less than 2 ml ideally.
I don't know what exactly is in your mystery pill. What fentanyl analogue is it? What is the solubility is alcohol? What binders exactly? It might work and it might not. Isopropyl extraction should work fairly well, but the only way to know for sure is to try it unless you can give more information somehow.
Edit: Shooting fentanyl is pretty worthless like Lolleedee said. A nothing rush and then you just fall asleep. That's my experience at least.