So I submitted the optional Harvard essay (mine was ~500 words), but I double spaced it, thinking that I should follow MLA format. I’ve been reading that they only want 1 page. Will my second page be cut off? Should I call them to change it?
No - they don’t care about formatting. Unless you need to update any of the information presented just leave it be.
The site probably puts the material into one format regardless of how it looks to you. I know that is true for a lot of the sites that involve students’ uploading of material. No matter what they put in, I see all in the same format.