2) Yes that is how WAR works. In the end, WAR attempts to determine how many wins a single player brings to their team over average replacement player. Perfect? Again, no, but it's a fantastic quick metric and I really didn't feel like spending 4 hours showing how silly of a comment you made. You can add 3 players up and get their cumulative WAR.
You are correct that you can add WAR up. The point is that you're comparing multiple players to one. If Detroit had keep Granderson, they would have used other players in the places of Schlereth, Scherzer, and Coke. So really, it's comparing the value of Granderson + 3 unknown players to the value of Jackson, Coke, Scherzer, & Schlereth. Which of those is better for Detroit.