I would suggest creating an API to bring in the data, then use the Call REST on a microflow. Then call that microflow on every startup.
Yes. Use the https://marketplace.mendix.com/link/component/72
Excel Importer Module. It is supported by Mendix and has a good track record of solving that specific issue.
Docs: https://docs.mendix.com/appstore/modules/excel-importer
If you are starting from scratch use the “App from Excel Sheet” in app creation.
in my opinion you will have to do it one by one