Good post.
I like McFadden. If they didn't draft Zeke, then I think Morris and McFadden would have been a good combo for #1 and #2. Morris the grinder and short yardage guy and McFadden the big play guy that is a good receiver and blocker; however, as you said, the #3 needs to play ST. I don't see any chance that 4 RBs would be active on game day; although, I guess they could consider Dunbar more of a WR.
If they can keep him on NFI (He has to still be injured to legitimately do it), then that's a decent option. I'm fairly certain that his salary does not become guaranteed if he is on the NFI like it would be if he was on the 53; although, they do have to pay him 1/17th of his salary each week he is on the NFI (706K for 6 weeks). Once the 6 weeks is up, the Cowboys can re-access their situation at RB. If McFadden is not needed, then some team will likely be hurting for a RB due to injuries and he could have more trade value.