Does the GL-AR300MD automatically determine which band the
host router it is connecting to is running at, or do you need to
manually switch between 2.5G and 5G?
It auto detect. You need to use firmware v3.03
But AR300MD will not be produced anymore. It will be replaced by a new modem soon.
Would you please share some rough ETA when to expect new model with ac/dual band support? 
We are having holiday to 9th Oct. So this is delayed a little bit. We will publish in middle Oct.
Sounds great, thank you for sharing!