I'm not a watchmaker myself but its easiest to think of your cal.12 & cal.15 movements in terms of a car towing a trailer. The chronograph module is piggy backed onto the automatic movement , when the chronograph function is not being used there is no extra load on the power unit. When the chrono is in use the power module has an extra load to tow around - just as a car tows a trailer. Clearly the heavier load makes for more wear.
As to your issue of a self starting chrono thats a new one for me. There is a more common trait where the chrono sticks at the 3-5 second mark when activated, due to lack of use or need for cleaning. Maybe a couple of the more experienced watchmakers out there have an answer for you.
By the way - i love the layout and odd styling of a Cal.15 Calculator - geek sheek at its best.
Cheers - Darren

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-: Darren--thanks.

: I do not have the seconds running normally as I am aware this is
: not good "policy." I do use the seconds occasionally,
: but pretty much remember to stop and return. The watches seem to
: keep time well, but from your response it sounds like a
: pinion/gearing issue where the normal movement of the train
: mistakenly will engage the chrono module--keep in mind that no
: matter how long I stare at the exploded diagram of the caliber
: 12, it becomes no clearer to me...