It seems easy enough to explain how most of this applies to
in particular? (I've looked in a few places and everyone agrees that this specific thing is the intended answer.) I think there are two clues. The first is in this bit:
My frettings loudly rush and ring
Above the people and most clearly sing
When I forth-fare on air
I'll quote a ...
And the wordplay is
I haven’t heard of that container indicator used before, but it does appear in this list of containers (apparently it can mean ‘breathe in’, thanks @jafe!).
The abbreviation also is very uncommon but does appear in things such as ‘pro-am’.
I would look at one guard’s shoes. If they were black ( or any other color ) I would ask if their shoes were black. If he says yes he is the truthful one. If he says no then he is the liar. I would then enter the truthful ones door.