Eltab
Lord of the Hidden Layer
I think you're taking player empowerment one step further than is really fair to the DM, Eltab. Players ignoring a DM's "setting canon" without meta-conversations about what that means is every bit as disrespectful as a DM overriding PC actions "because your PC wouldn't do that". Players dictating setting background without building inter-player and player-DM trust is a mistake.
But we absolutely agree that the best way to start dealing with the situation is by having a direct conversation about it. Then joint decisions can be made.
I probably should have written my original post upside-down to how I really did: start with 'talk to your players', then 'this is setting up for problems'.
The metaplot I could see being invoked here - and I should have referenced it - is something like Wotan in Wagner's Ring Cycle: for some reason, he needs to create a hero who will work his will BUT he cannot be connected to the desired deed or be held responsible for the consequences, SO he sets the individual up to be cursed by the gods before he is ready to perform the deed. In this case, to get what he really wants, Gruumsh needs an anti-hero who does not behave much like an orc - the PC's origin as described in OP is a person practically best-choice to (unwittingly) volunteer, so Gruumsh takes advantage of the opportunity. Now the hard part: what does Gruumsh fervently want, that he can't get via his usual 'Hulk Smash' MO?
Another thought is drawn from IRL: in 1945, France and Germany had a 300-year history of going after each other hammer-and-tongs once per generation. After 1945, both nations found something more important to do (rebuild after WW2) and a more threatening enemy (USSR) so they were willing to work together. 4e Plane Above: The Astral Sea suggested that the gods have too much on their plate and they really need reinforcements ... and/or to quit frittering away their strength kicking each other in the shins. Gruumsh and Corellon might agree to back off in the face of some other problem, and coincidentally the PCs happen to be their chosen group of problem-solvers. What might that problem be? The group can cogitate on that a bit. Maybe Asmodeus is stealing souls that belong to the other gods; maybe the Far Realm threatens to burst into the universe; maybe a number of Primordials are going to break loose rapid-fire and even the gods cannot be everywhere at once.