A couple of thoughts: You may well find that the importer tells you that this degree of divergence is within specification, but I guess it is worth a shot trying to get them to replace it.
If they do knock you back, the first thing I'd do is consider the way you are assessing POI. For a shotgun to be used on moving critters, the way to assess POI is to put an aiming mark on the target, stand up, focus on the aiming mark, mount and fire. IMHO it is a mistake to do it by resting the gun and aiming along the rib,if that is how you've been doing it, because that won't necessarily tell you where it will hit when you mount and fire normally. As well, by holding it down on a rest, you affect the way it moves between the firing pin striking the primer and the shot charge leaving the muzzle - even a .410 will move.
I'd also try it with the loads you want to use - maybe try a few different cartridges to see whether that makes a difference. I'd also get the boys who are going to use the gun to try it.
You could also bring to POI to the left a bit by altering the stock. If you are right handed, a bit of extra thickness to the left side of the comb (not the top) of the butt where your cheek meets it. That way rather than having the right barrel spot on and the left 6" hitting to the right you could have the right barrel 3" to the left, the left barrel 3" to the right, which is not even worth worrying about. Of course if it is to be used by your son/s, this should be done with them shooting, so the gun hits where they look. You could easily do this with an expedient like some cardboard taped in place, to see if it will do the trick, without making any permanent alterations.
As far as altering the chokes is concerned, I think the bores are chrome lined, so this may not be as easy as it sounds..
