Villagers can be killed, but not permanently (death revives them the next day). I need to make this clearer by adding a gravestone where the NPC dies and you can ask Death about it there. Iris should re-appear the next day, unless she dies again (if a lot of afowl make nests in her territory and attack her as soon as she awakens, it's possible). I'd watch her location (in the relationships tab), and if you go to her location and there's nothing there, she died that day.
The error has to do with your save file quota, not memory, so it can't have anything to do with other tabs being open. But, I did realize that the quota is set per domain - in this case itch.io. So I don't get 5-10mb for Witherspring Wilds, itch.io gets 5-10 mb for all browser games and site cookies total - if you play a lot of games on itch.io, that could easily fill up! A simple solution would be to play on a site other than itch.io (like our homepage), or to clear cookies on itch.io to reset your quota (but beware, this will delete ALL your save files across all games on itch.io, so make sure to back saves up before you clear cookies).