1. Quit worrying about what is the best penetrating product or weird homemade formulas and just go get the PB Blaster. It's real hard to find - Not. Auto parts place, Home Depot, Lowe's, Walmart, hardware store, etc. PB should be in your standard maintenance supplies.
2. Soak the screw over night. As another poster advised, wrap a soaked rag over the screw if needed to keep the screw wet with PB.
3. Call in a machinist. You don't appear to have the tools or experience right now to deal with this. Look over their shoulder and learn for next time.
4. Time is $$$. How much time have you spent on this? How many $$$ are lost by machine downtime? If qualified staff and tools are not on-site then you need to have both a machinist and an industrial electrician on call 24/7 to keep the machines running. Stuff breaks. Stuff repaired or maintained by persons outside their skill levels is a recipe for more breaks and safety hazards.
No need for that, friend. I've been a tradesman for forty years, and have done a fair amount of wrenching in my days. I just never ran into a set of screws so tightly bound before.
This really ain't rocket science, and I'll probably have the whole assembly apart by tomorrow. In the meantime, my machine is still running on its other two drives. I'll get the third one back online by this weekend at the latest.
I do appreciate the concern and good advice.