If your C is assessed as a fatal accident before you forget your helmet, then forgetting your helmet does not increase C....
Basically its a trade off - you go light and decrease T, exposure time, in return for accepting some increased hazard (PxC), in order to reduce risk.
Case in point: Slipstream. You have at least two hazards: death from fall, and death from avalanche. Yet you go light, and solo most of it, vastly increasing your exposure to death from falling, in order to reduce your exposure to death by avalanche - "speed is safety" and better the 2 hr Twight solo than the 20 hour belay every pitch ascent.