"A “diplomatic agent” is the head of the mission or a member of the diplomatic staff of the mission;"

"1.A diplomatic agent shall enjoy immunity from the criminal jurisdiction of the receiving State. He shall also enjoy immunity from its civil and administrative jurisdiction, except in the case of:"...

Michael Kovrig was not part of a diplomatic mission to China by Canada. Therefore he is not a diplomatic agent.

This is a clear cut case. Ex-diplomats with private passports are not entitled to diplomatic immunity.