For your first question, based on the needs that you mentioned, I think the CDS Entity would be a better solution for you. If you have PowerApps Plan 1 or Plan 2 license, you would be able to use CDS functionality in PowerApps.
In addition, the CDS acts as a embedded data source within PowerApps, which would have high process speed than other data source.
For your third question, Common Data Service has a rich security model to protect the data integrity and privacy of users while promoting efficient data access and collaboration. You can combine business units, role-based security, record-based security, and field-based security to define the overall access to information that users have in a Common Data Service environment.