1) No. Good idea though. So you're suggesting I attach a meter to the batteries during an exposure and check the voltage. I think I can do that with these 76's.

2) Yes, it must. The timing is electronically controlled. (There is a 16s setting).

3) It's a prism finder that uses the camera batteries to meter. (The Bronica AE prism finder)

Anything to inspire more troubleshooting is useful.

I took a photograph without the prism finder, set to 8S, and it worked. So what's a real head scratcher is why 4s works but it seems to completely ignore 8s. I guess my next test is to shoot the prism finder at 16s and see if it times it correctly. If not I can presume that the prism finder is unable to draw enough power from the batteries > 4s to close the shutter and as such is defaulting to 1/500.