[HighAltitude]

After lots of experimentation, I was finally able to enable 3 flight modes (using my new Spektrum DX9).  I had configured the "Gear" switch (Channel 5 on the transmitter) as my arming switch configured as Accessory 0, but every time I changed anything from the default values, the arming switch would quit working.  I'm not sure what sequence/combination of configuration changes I made to my transmitter and Librepilot, but the RC Input now shows input for 3 flight modes using the transmitter switch D as Accessory 1. 
I still can't change anything in the Assisted Control column without disabling my arming switch
I'm still confused about the definition of "Settings Bank" and what its settings mean.
Is there a tutorial somewhere that might be more helpful than the basic help screens?

[nitromethane]

...but every time I changed anything from the default values, the arming switch would quit working...

...I still can't change anything in the Assisted Control column without disabling my arming switch...

I'm still confused about the definition of "Settings Bank" and what its settings mean.

I'm not quite sure, what you're trying to set up (a screenshot would be helpful), but some flight modes will prevent you from arming for safety reasons.
For example: you don't want to arm the copter in position hold and have it imediately jump of the ground

Settings Bank enables you to apply different PIDs to different flight modes, take a look at the stabilization settings page, you'll see you can configure 3 different PID settings banks