What puzzles me is that you get battery voltage on both sides of those main fuses and still you get a very low voltage on the connector supply pins on ignition switch when the problem occurs. It's also a shame that BMW wiring diagrams don't reveal everything on the wiring and connectors on front fuse box. To me this starts to sound like there is something internally wrong on the front fuse box. Why don't you try and remove connector X10459 and see what happens. Switched Terminal 15 and 30 voltage supplies are routed via that connector back from ignition switch to front fuse board. Unfortunately the diagrams don't tell what else is connected via that 10 pin connector.
EDIT: After digging at least fuel pump and body module (GM3) get's there supply via that same connector so those will drop out. But what happens to other electrics?
