Well, iirc, E0 encompasses everything before E1. There are no negative eras because all of that time was in Era Zero.
I agree that this should be canonical. There wouldn’t really be a point to it otherwise.
Cassie spun around and almost burned a tortilla; she hadn’t realized he was still in the room. “Uncle James! Hey! Um. Jace and Ax, they’re getting married.” @Dr. Dapper
This wouldn’t be Era 0, that’s more of an OOC thing though it might not be Era 4 either, as it might go past when E4 ends. A bit like the fantasy/horror/Scadrial threads, outside of the timeline.
They went to several stores and had magical side adventures and it was super cool and then they came home with the stuff as planned with no Voidbringer attacks or police interventions or lengthy conversations or getting ready for prom or backstory or scary Unmade dreams or extra weddings.
Cassie parked the car in the garage and dumped the stuff on the kitchen table for Evelyn. “How’s it going?”