"Mainstream"

I think Shamala/Nightmare, Puppeteer, and any other quest chain longer than 3 quests should be on it's own page regardless of weather we decide to call it mainstream or not, because throwing it on something like the sidequests page might make it look like the individual quests can be done out of order to someone who hasn't done it yet, and putting "finish the previous quest" in the "how to get the quest" box for each one of them would be messy and tedious.

Anyway, about your formal definition of mainstream, I'm not sure "first quest chain of the update" makes sense; Saga episodes 2-10 aren't the first quest chain of the update, unless you're counting updates smaller than a gen which would include Shamala, Nightmare and Puppeteer.