Saskatoon, commanders are authorised to "vary" SOPs in some circumstances, such as waiting for a rain shower to pass.
I would argue that they are only authorised to 'vary SOP's' if by doing so they increase the safety of the operation.
So the question then becomes "Did breaking this well known and easily understood SOP increase the safety of the flight?"
The ops manual may be written in black ink on white paper, but the world is not black and white.
It's fairly black and white on this one. Basically it's "Do not depart less than 30 mins prior to ECT due to the need for visual maneouvreing below MSA in the case of an engine failure on departure"
I find it hard to see how you can argue against that unless you start pointing the finger at management and undue pressures to get the job done. Personally, I think that is what needs to be done.