By that point, the case had been going through the courts for some years, financed by the children of one of Scott’s earlier owners who had turned against slavery. They lived in Missouri. So no, the case wasn’t hatched in Massachusetts, and Chaffee wasn’t any kind of mastermind.
Chaffee (or his wife) could have stopped the case by freeing Dred Scott, but arguably they would have faced more flak for making the case moot than they would have for owning a slave. After the decision, they did turn over the Scotts to the Blows who had financed the case and Henry Taylor Blow freed them.
This is my point. A political propaganda coup is more important than the actual life of the slave.