I want to support multiple windows, just not multiple instances of the SAME window. Can I check for a window's existence using SwiftUI? If so, how could I bring it forward?

Exactly, if you want to use CloudKit integration, you have to have to make sure every var in the model is either optional or has a default value. Moreover, it doesn't seem that you can turn CloudKit integration off without entirely removing the entitlement from the app. I was trying to use CKSyncEngine, but that is currently not possible, from my explorations.

Thanks, @sha921, I saw that note, but I assumed that applied to newly opened windows. If the user has multiple windows open when they quit the app, surely they're not all re-opened one-on-top-of-the-next at re-launch time?

Yep, I missed that in the release notes. That does indeed work, but not if you want to use CloudKit (unless you want to make all your fields optional).