I would advise to first get the file from S3 (as you are doing here) and cache this in your Mendix database. As a next step, you could use a TaskQueue to fire a task in which you process the Excel import. Of course, both with proper error handling.
If your template has been setup correctly (please have a look at the Excel Importer documentation for this), you can use import logic as shown in IVK_ImportTemplateDocument in the Excel Importer module.
Hopefully, this helps!