The whole point of organising a letter campaign is for an influx of letters to be sent in around a close enough time frame to send them the message that a lot of people do want this series back. Whether the feedback from us ends up paying off in the end, we'll see
I think this as well. Assuming that half of the base newcomers are clones, they had to add characters where they can reuse assets and moves in order to make the newcomer count high enough as I assume they didn't have the time to add as much. All the newcomers we did get have their reasons for their inclusion which I can see why they took priority over Isaac, but judging by how much Golden Sun content we got as consolation I'm confident he was a pretty high runner up for inclusionI remember all those posts on the website were numbered and Isabelle's didn't quite match up with
Personally, I think that if Ultimate had the normal amount of newcomers and didn't include every veteran, Isaac would've made it, but that's pure speculation and hope.