Capacitive voltage drop - surge and safety
Well, I'll answer my own question, partially at least.
After a lot of searching it seems reasonable to assume that a residential AC mains peak can safely be assumed to be a 8/20µS peak with 6000V peak. It is also reasonable to assume the current peak will be a maximum of 6000A.
With regard to the additional protection device, I'm still not aware of series devices that could serve the purpose. An alternative would otherwise be a MOV at the input instead of a TVS across opto-coupler LED and resistor. Clamping voltage will be higher (worse dynamic behavior) but in return they will allow much higher energy absorption.
All in all, I will abandon this whole approach in favor of another energy efficient technique that I found on this very platform (but found through Google, not with the SE search function):
Efficient method for isolated detection of AC voltage without a stepdown transformer

Shame on the question poster for not accepting the excellent answer found there.