Disjunctive Graph why directed

[MagMueller]

I can see the point to use directed arcs to resemble the sequence of operations in each job. But later you Calculate the score for an operation at the beginning of the job, so the information from later operations of this job could also be helpful to get the true priority.
So maybe undirected arcs could improve the performance further. To distinguish them from the machine arcs maybe you could use different type of arcs to update the nodes in the graph.