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.
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