I think the best way to conquer the JR is to do pretty much what they are doing, but with a successful truce with the Tau. Ebongrave would have to go - making him a Rogue Trader as a 'reward' for his service should suffice. That'll free up a huge amount of resources (both Imperial and Tau) that can be turned against the 'nids. Once the 'nids are dealt with the Imperium will once again have the resources of the Orpheus Salient (what's left of it) to draw upon (and some of the forces there can be redirected) and can finally crush the forces of Chaos. From there they can move on to mop up the Tau.
Ebongrave is essentially the greatest impedement to Imperial victory.