From practical experience, I can confim that the shape of the area in the narrow route brief is actually a "lozenge". That is why when I fly say from Oxford to Southampton, I get notam warnings for things on the isle of wight (Kite Flying) which is beyond my destination.
With regard to knowing what is happening 10nm beyond the destination there is a total difference between this and the calculation of safe hight to fly.
We calculate a safe flight to fly at (VFR) or minimum safe altitude (IFR) which includes a distance beyond our destination or turn point so that if we unknowingly fly beyond out destination we will be safe.
However, even if we know that there is a TRA 10nm beyond our destination, if we don't know that we have passed the destination in error, we don't know that we are about to enter the TRA and thus the knowledge at that point is wasted.
Knowledge is only useful when it can be put to good use!
Regards,
DFC