SWISS tend to have good availability in C as do LH, but this might mean a track down to MAN, or alternatively an early morning flight to DUB with FR, if one exists.

To be honest, I've struggled to find availability to/from Canada at all. Seems to be very tightly controlled in C by AC, the only thing I could suggest is look at splitting your party up, going 3 on one flight, 2 on another, etc, or booking c where it is available, booking the rest in Y and paying the 20 quid fee to change to c if/as it becomes available.

There is a post on another thread of a person finding 7 seats on LX in C, so it is possible!

Best of luck!