You got that right, a backgrounding diet that keeps calves green is hard to do with corn silage and distillers.

The actual NEg target will depend on how fast you want to get them to 900, and the weather and lot conditions they will be in, but 44-46 would be a good starting point - that should easily support about 2.7# of gain if the winter isn't terrible. I actually do 40-42 NEg more often. 

If you grind them fine enough (so they don't get sorted) corn stalks can work, but they are really bulky in a grower ration and will limit how much you can get into the mixer. Grass hay is the best. You could also use some rye silage (if you or a neighbor had it) to dilute some of that energy.

Protein requirements go down as the gain goal goes down. 11.5-12% would be enough at that energy level - the implant actually makes them more efficient in how they convert protein into gain.