Updating from outside of AppSheet?

Data is stored in Excel on SharePoint. If we have a ton of new records to add, can we simply copy and paste into the Worksheet or will that mess things up?

You can copy and paste that data into the sheet. The users will need to sync their app before they can see that data though. If you have any special rules for cells and you are pasting data that does not follow those rules, that might cause some errors for the users but will not break the app.