I'll play around some more, but on the Frontier 90, I added a Zebra to my cart and it applied the discount.
Maybe the previous weirdness was because the other colors were from third party sellers. The event only applies to items sold by Amazon.
As for Mom and Prime, Mom included Prime shipping (but not video, I think) when it was in beta, and you needed invitations to join/membership was limited. The discount used to be greater, as well (I think 30%).
Now it includes a trial of Prime, but you have to renew Prime to keep your benefits after the 3 months is up. Mom is basically a free add-on to Prime that gets you the 20% S&S diapers and wipes and also 20% S&S every month you have 5 or more S&S items sent. Plus, obviously, access to these events.