Quick update: my cycles usually last 4-6 weeks. I had one that started around December 9 (Thankfully, my first cycle in nearly two years.) Between 12/9 and 12/20 I probably had ten headaches. On 12/20, I took the first month dosage of Emgality. After that, I had four more headaches from 12/21 and 12/24. None since then.
So, who knows if it was the Emgality that made the cycle shorter? Probably too soon to tell. But, so far so good.