You can, but it works a little differently than the channel PFLs. You choose the listen button on the main LR fader and then adjust the volume on the home page. Kinda stupid.
Yes, it’s only ever possible to listen one channel at a time.
If you actually mean to listen when the master fader closed, that’s not possible, as the signals from the mixes are always listen after the fader.
A possible workaround would be if you had still a spare out mix available - or better yet, a pair for stereo.
With this, you could create an additional post-fader mix of all inputs and returns and then listen this “duplicate” of your Main LR mix, regardless of your Main master fader’s position.