MobileSheets does not have an API, and is unfortunately limited by the restrictions placed on UWP applications. You can't load it easily with command line parameters and it also will not have access to any local files that were not selected using Microsoft's file picker. One of my long term goals is to switch the app over to Avalonia so it will be a normal desktop application without all these restrictions. At that point in time, it would open up a lot more possibilities for automation. There is no automatic synchronization available either. The closest thing is the library synchronization feature, but this won't work for what you are trying to do. You can batch import an entire cloud or local folder if desired and MobileSheets will ignore files that are already in the library.
Mike
Mike