Yes, and since you've wiggled the wire, we've learned there's nothing wrong with your wiring.Is the blue wire that's connected from the b2 button to the Jamma connector supposed to *beep* without pressing the button?
1 on your meter is the same as OL on mine, perfectly normal. And this shows there's nothing wrong with your button.When set to continuity, my multimeter shows a number of "1." When touching the two spade connectors with each probe, and pressing the button, it begins to *beep* and the number drops to 0. I tested this on two other buttons and the same exact thing happens.
Your problem very likely lies within the Mistercade. Whether that's some weird thing wrong with it, or it not receiving the right amount of power.
Let's next turn on the cabinet, set your DMM to DCV and put the red lead on the 5V jamma pin, and the black one on GND next to the 5v pins. Note the voltage, then repeat with red on 12v and black still on the same GND. Let's find out how that goes next.