Just looking at a map and it's scale there is about 60 Km from mouth of Attawapiskat river to Albany and about 80 Km more to Moose river ( Moosonee, location of railway south). From the Ring to Attawapiskat is also about 150 Km.

Total distance is about 300 Km, some of that distance could be reduced if one bypasses Attawapiskat and takes a more direct route to Moosonee.
My point is that the cost of this additional railway should be more expensive than KWG's shortest north/south route because of longer distance and more northern location.

However you are right on the money when you say Mattawa could lose a golden goose.

Greed is sometimes self defeating. Cheers and Thanks for that heads up, Ed.