Good info!
"IIRC some singles have the exhaust offset laterally to give slight assistance with torque control"
Has that exhaust pressure ever been used to assist a puller tail rotor? (So tail rotor not in exhaust stream) Might even be possible to use coanda effect to help suck in cool air, maybe a fenestron design, reducing tail rotor load further. In practice the duct losses would probably lose any advantages in tail rotor power reduction though...
"...how long it would take an average human being to breathe the same air a helicoptere engine uses..."
Might interest you to know that a cyclist's conversion efficiency is determined at 20% (athelete or not). Probably explain's why you get so hot cycling...
Mart