You do not need to load the html file onto Moodle. Save the html file on your pc in a folder of it's own. In Storyline insert a web object into the last slide, instead of entering a URL, click on the yellow folder and navigate to the folder where you saved the html file.
This path will be included in the web object. Publish your Storyline project as per normal.
Of course you before you do this - you need to edit the html file to include the Moodle course page that you want to return back to.
With the last slide in Storyline I added an additional one with some text... "Please wait loading page...." and put the web object on this. slide rather than the last one of my project.