The Vol- button might be pressed the whole time, it does not matter if it is ever released, it just has to be pressed upon K3 reset. As I said before, I think I'd just tape it down when timing is unclear.
It also seems I had installed two drivers that came with ATK: the MX35 one plus the "windriver", by running "wd_install_4NT.bat". Not sure if both are needed, but I think the latter might provide functions for the other one.
I would certainly not risk a K3 that is "in production use" and not broken :-)