[Max Lobovsky]

19 Nm was a typo, I used 13Nm (post edited). With the reduced length of key engagement, you are down to a safety factor of 1.53. That definitley sounds like its cutting it too close. A relatively simple solution that I know is pretty common is to use a double keyway, a key on opposite sides of the gear. It wouldn't be too hard to broach the gear like this (aligning the second cut may be difficult), but I don't think I have seen any stock shafts that come double keyed.