As far as oven hardening, you'll get even results, whereas quenched as they come out of the mould, will result in varrying hardness from about 20 brinel to 30 brinel.

Since Linotype is about 21/22 brinel, the quenched WW should be just fine. One thing about the WW, is they don't get near as brittle as Lino is. Your recovered balls show that.

As to shooting apart, seems a minor change in load would bring them together?

Forsyth suggested sighting for one barrel and learning to hold for the other.

He also stated barrels should shoot paralell to the axis, centre of group. That way, they'll always shoot to the sights, albeit an inch and a bit wide of each other, but parallel out to maximum range, neither diverging nor converging. Yours are so close, I might be inclined to sioght for the right barrel, or whichever is fired first.
Sighted as Forsyth suggested, one would only have to hold 5" or so for the 'off' barrel at 100 yards - pretty simple.

BTW - what load did you use, please? E-mail or PM if you don't want to publish it to the net. I realize this is a 12 bore nitro Rifle, not a shotgun proofed for normal 11,000PSI shotshells.