Only seeking more profesisonal answers than i can find in my study guides
This coffin corner may be referred to as 'buffet boundary' perhaps? May be slightly different. Buffet boundary is a position on the corner of the V-n (flight envelope) diagram where positive and negative load factors are limited due to turbulent airflow on the tailplane exacerbating the airframe loading.
The thin air answer just seemed too easy. Perhaps That is all i need. I also wonder whether less manoeuverable means that is is 'physically' less manoueverable (ie cant control) or less manouverable in the sense that it can't be safely manoeuvered without encountering high speed buffet regions. Hmm. More thought required.