Corporate Owned Enrollment - Successful enrollment despite modification of data in the primary user

I think it depends on when you break out, because indeed it's possible to do so during the process. As long as no accounts have been added, it'll function fine if DO hasn't been set at the point of breakout. If after DO and you modify accounts .. you're adding additional accounts. 

The user data doesn't really come into it, so download what you want, adjust apps, whatever. No issue. It's the same requirement for setting DO via ADB after setup completes. 

The EMM should account for this, for example AMAPI removing any user-installed apps that aren't permitted by policy in the policy-managed areas, but there's going to be more flexibility with COPE since the DPC won't be actively maintaining inventory there beyond some explicit, by-packagename policies applied to the parent.. that said global blocking of sideloading and such can happen by policy to prevent further issues.. 

But it happens.. I can break out of fully managed enrolment on some devices also, though default policies are restrictive 🙂