My 2p worth...first treat it as you would a motor vehicle that is "unfit for purpose". That is you must REJECT IT outright for if you allow the gunshop to repair it then you have accepted the item and can, I believe, no longer then reject it.
But I would before that try it with its iron sights or, even, some auxiliary sights just made from stiff cardboard, or a bent piece of tin can, and taped to the barrel with sellotape or masking tape. This will allow you to tell once and for all if the rifle can group or if, in fact, the problem is in the 'scope mounts or the 'scope rail. It doesn't matter with your auxiliary sights (if it is not fitted with iron sights) about the MPI as all you are initially interested in is the group size and not the group placement.
Also I believe that it helps all if you name and shame the gunshop responsible so that others may at least profit from your misfortune. There is a lot of acceptance that, somehow, a gun isn't subject to the same consumer legislation that all other items are subject to.
I'd say that a gun that cannot (as say a stalking rifle group into 3" or 4" at 100 yards isn't acceptable) and that a .22LR rifle that can't group into 1" to 2" at 50 yards ditto. Quite simply because it would not be humane to use such a weapon against live quarry.
Have you quoted the serial number of the gun to Viking Arms to ask if there are any known issues with that actual individual rifle and, I assume, is it the original barrel, etc., that is on it or has the gun been "messed about with"?
And I think that others have mentioned taking the moderator off for your testing? Lastly ask Viking if the gun was screw cut by them on import or not. As if it was not, but done after, there may be issues of the gun now being out of proof and thus exposing the RFD to a large fine under the Proof Act.