Hi Andreas,
yes, unlike on the Airbus, a lot of stuff hinges on the battery (like the power to the generator breakers), and switching it OFF will leave you with nothing but the hot battery bus (and ground service, if there is a GPU plugged in and the switch is toggled - this is not modeled yet). Onlike Airbus, Boeing relies on their pilots to have a brain and not turn off the battery switch until they have a VERY good reason to do so and understand the consequences ;-)
The IRS units have DC backup, but not on the hot-battery bus, so they will die, too. Pressurization should be able to be regained, you need to get the cabin back into parameters first with the Standby or Manual mode, then you can re-engage automatic mode (switch to standy and back).
You can re-align the IRS in attitude mode, but the position will be lost - however the GPS will work, so that is not the end of the world.
The "huge speed" inthe MCP speed window is definitely a bug, some situation that you guys are getting yourself into that we haven´t tested for... I will try to recreate this myself and then we put some sort of stopgap to the maximum speed. I am a little wary of tinkering with that code too much, Tom and I want to really rip the guts out of VNAV and then sow everything back in neatly in a future update, and this will likely affect many of these quirky speed calculations...
Cheers, Jan