Hi @Mads-G,
The MIDI button is part of something that we call the ‘legacy’ MIDI mode internally. In this mode, every MIDI device needs to be enabled manually through this menu. Starting from 1.7 we added a MIDI aggregator that combines all MIDI inputs from every device to a single port. This means there is no need to connect to the computer any more when changing MIDI keyboard. (however, we will be bringing back this ‘legacy’ mode in a future update).
regarding the display brightness / led turning on, this seems like the display&knob controller of the devices sadly crashed 
Would you happen to have more info about what sequence of events causes this? These are the nasty bugs we would like to fix as soon as possible. Feel free to PM me about this as well!
When the DuoX gets connected to any PC it identifies itself as a network device in normal operation. So it’s expected that it does not show up as mass storage. There are some possibilities here for the future, but we first need to get to a stable device before we invest in these new things. for now, this might interest you Duo X as USB Audio input and MIDI device | EXPERIMENTAL
all feedback is greatly appreciated and I hope this info helps!