I would love to use Marginnote for professional work in the future, i.e. reviewing legal cases and making the excepts of the facts of a case right within the app. In order to do that I would need some more information on how files are being handled and what is being transmitted.
From what I understand it is possible to not store the documents within the Marginnote folder in iCloud and still work with them (just not on multiple devices) as well as turning off iCloud for individual notebooks.
Ideally, you would write out a Data Protection Policy and name a Data Protection Officer. I am sure that you could charge additionally for MarginNote 3 Pro that includes more granular control. I’d be happy to make some suggestions once you have to time to look into this.