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I assume they are on iMessage as well at some level are they not?

If any party receiving an iMessage uses iCloud backup, it should be available to the US govt one way or another.

Am I correct that parties who don’t use backup are indeed messaging “entirely” privately, barring Harvest Now, Decrypt Later?

iMessage is actually post-quantum prepared - see https://security.apple.com/blog/imessage-pq3/

Not to my knowledge.

iMessage keys never leave your device until you back them up to iCloud.