honestly, there is no harm jumping the 5V, all you are doing is by passing the blanking signal, if game boots, it is the blanking.

Another oddity with the Wms sys is with the 6821 PIA chips. Sometimes these dont just fail automatically. I chased a fault for ages once scratching my head.

Test the output voltages on the CPU PIA's. If you have anything under 4V this is not seen as a logic high and will cause the CPU to lock up. If you find on the outputs any voltage around 3V then replace this PIA.
I chased and chased a fault with a logic probe and came up with nothing. tested with DMM and got 3V on one of the outputs, so then i wondered - is this enough for logic high?

Replaced PIA - got 4V or so and game booted.

Worth a check