Can I access and run SEPAL activities through an API?

Sepal has an API that can do anything you can do though the GUI, but it’s neither supported nor documented. It’s the API your browser use to talk to the Sepal server. It might change at any point, though chances are that it will stay pretty stable.

I would suggest that you perform whatever operations you want to automate inside the Sepal GUI, with the browser’s developer console open. You can then inspect the requests/responses made. Any API call support HTTP basic authentication headers.