The reason they're not going to be on the Calamity Salvager is because that would make it too easy for players to open up their own "Mogstation" in-game to sell the seasonal event items. Invite someone to be tenant, they remove the items they want and pay you, you go back to the Calamity Salvager to buy more, etc.

Personally I wish they had not bothered putting them on the cash shop and instead would give players a way to earn them (or rebuy them) each time the related seasonal event returned to the game. I miss some of the mini-games we used to have with the seasonal events.

Why do we only have 35 days after a house is demolished to reclaim them? It's only speculation on my part but I suspect it's a database issue related to housing items being registered to the plot and not the player, and then the plot or previous ownership of it being linked to the player. They want to make certain the items are gone from the Resident Caretaker before the plot ends up demolished again and a new set of items added. Personally I think they could extend the hold to 60 days safely but who knows what lurks in the heart of their coding to prevent that.

What I do since there's no telling when a real life emergency might happen is store all of my seasonal event items either in my apartment or on a retainer (outdoor items) if they're not currently being used in my house. That way if I can't play and end up losing my house, most of the hardest to replace items are safe.