Very well said Simon. As an old school analogue audio studio person I'm not all that familiar with all the various DAW situations which is why I avoided mentioning them, but thanks to you I'm learning fast.
As for ideal listening level when mixing I was taught during my Diploma in audio engineering training that 70dBA to 80dBA is the "sweet range" for most mixing endeavours, however where required and depending on the recorded material critical listening tests of completed mixes should also be done at 60dBA for 'background music' situations and for short durations at 90dBA to 100dBA for party/night club scenarios...
It is a widely help misconception that all radio stations keep their music libraries in MP3 format. The four local commercial radio stations that I do contract technical work for all hold their music libraries on local HDD's in predominately AAC format, and all their online stream(s) is done via a digital split at the studio so arguably the quality of the stream is vastly superior to what is broadcast over the airwaves. Except for the microphones and loudspeakers the studios themselves are all digital environments... And as the stations are networked if any one station looses its local HDD library then it will "borrow" music over the network via VPN from a sister station until the local HDD and library is restored, and neither the DJ nor the listener is ever aware of where or which HDD the music track itself is actually sourced from...