Hope you're doing well. Per the description, I understand that you want to know the security levels of your documents stored with Document Cloud.
Since Adobe Scan works based out of Document Cloud servers, you can refer to the below link, which would answer your questions 1-4:
Answering question 5:
It depends on your requirement and how you want to create the document. You can create a new document for every scan or select multiple scans and create a single document.
Answering question 6:
The same account can be used on multiple devices to access files.
Hope we were able to answer all your questions.