I think Phibes basically takes the view that if the ten plagues were good enough for God (as a means of taking revenge), then they're good enough for him.
Yes, and there is one point where he proclaims to a Doctor that "God is on My Side!!"
He felt he wanted to extend God's Wrath for the death of his Wife, and the Plagues of Egypt seemed apt somehow.
My deal with the Plagues is that I felt the filmmakers changed some of them unnecessarily. For instance, there was no Plague of Bats, it was a Plague of Flies. And "The Beast" was actually the Death of the Egyptian's Cattle & Livestock. This movie has the Plague of Darkness as Last and happening after the Death of the Firstborn. Actually, those two were switched and the Death of the First Born was the final curse. That was the one that made the Pharoh finally give up and release the Israelites.
Yes, I realize that some of these had to be changed to make for a more interesting demise for the Doctors, but I didn't feel they needed to be changed that much.
Steve, I thought about that too. But remember the scene with the rabbi? He says that scholars have been debating the exact order of the plagues for centuries. Not true, to the best of my knowledge, but a convenient out for the filmmakers to put them in whatever order suited things best.