Specifically, Korvold's ability would read "When(ever?) you sacrifice one or more permanents", triggering once for the sacrificing, instead of once per permanent.

This is mentioned explicitly in the design article: magic.wizards.com/en/articles/archive/card-preview/… "(Also, yes, these do work really well with partner commanders like those found in Battlebond!)"

Note that the rules will change with SOI release: The discard into exile is now mandatory, and choosing not to cast the card will put it into your graveyard. This has no effect on the rest of the answer, you can still 'process' the card before it can be cast. Source: magic.wizards.com/en/articles/archive/feature/…

Sort of. You can put decklists in Deckbox, and see how many of each card is in your inventory. Unfortunately, you cannot 'tag' cards to see in which deck they are, or how many you are using across your decks.