Bally schematics are confusing because they separate the cabinet wiring from the playfield. To make it easier to troubleshoot I pencil in the coin and credit button switches into the playfield schematic which normally shows them as blank positions.

The credit switch is at the intersection of ST0 and I5. Since the credit button is in the cabinet looking at the cabinet schematic above the wires go to to J3-2 is ST0, and J3-14 is I5.

So here is your puzzle. Pencil in the cabinet switches into the blank positions on the playfield matrix. Now read the matrix like a road map. You need to drive from S0 to via the credit button to I4. If that route forces you to go backwards through a diode there's your problem.