You have Iran entering the war o the idea that Baloch separatism would spill over into Sistan Balochistan or whatever it was called then?
The shah had a working relationship with India, and off the books Israel. Now if Mossedegh had succeeded, another story.
I'm afraid I can't see the motive for Iran to join in. All I can think of is if the Iraqi embassy in Iran had weapons intended for Khuzestani rebels, and I don't see how that would work.
Well, ok then, but what if it was just Pakistan vs Iraq?