The use of a January date in your example does give one pause....
I agree that if your plan provides that comp is based on the default rule in 415, severance should not be included whether it was paid pre- or post-termination of employment as that comp definition includes only amounts received for services actually rendered. (I didn't use quote marks as I have not busted out the books but I think that is right). This rule would not apply to NQDC, so timing seems relevant.
However, I am not certain about your blanket statement that severance is never included when using the W-2 6041 definitions. I believe severance and NQDC are includible in W-2 compensation and are reportable, including for purposes of the 415 safe-harbor definitions. Can you provide authority for your blanket statement?