My ACC button is in, and I have a D7100. I use a fibre-optic connector. There is no problem with the flash synchronising with the camera, its just the strength of the flash that is an issue. I am not aware of a problem with the ACC button. I use the flash in manual mode.
I have also undertaken a test with both flash guns in exactly the same settings and position, taking shots alternatively - there is definitely an issue with the output of this gun.
Please can you explain why a faulty magnet on the ACC button might affect the strength output/control if the flash is firing? That seems a bit odd.