What's the easiest way to get a references section into a single citation format if they vary to begin with? I have a ~5 page references section with citations added by various authors in varying formats. What's the easiest way to get them all into the same format?

The two options I see are to do it manually or use citation management software like Zotero. Doing it manually is feasible but tedious, prone to mistakes and will probably take an hour or more. I'm not sure if it's possible to import these varying format citations into Zotero and export it in a common format, but that's what I'm about to try.

I wanted to check with experts here first. If I find Zotero works or not I'll add that info to this Q&amp;A.
Doing this in a reference management software (I use Zotero, which you mention, but there are others) is absolutely the way to go. Of course, setting up the reference database for one paper may take the same amount of time as manually reformatting, and learning how to use it in the process may take longer. The advantage is in reusability. For any single paper the up front cost may not be worth it, but the time savings on paper number 2 and beyond is incredible.

Unfortunately you won't just be able to paste in the formatted citations and have Zotero recognize them (so far as I know). Zotero does have a way to add items by an identifier, so if your coauthors included DOIs, adding the references should move quickly. Additionally, if your coauthors used any reference management software themselves, you can ask them to export it to a common format (BibTeX is pretty commonly supported) and import those files.

If worse comes to worse, searching for each reference title in Google Scholar and using Zotero's automatic ingestion should also be pretty quick.

Zotero's documentation on these various ways to create items:

https://www.zotero.org/support/getting_stuff_into_your_library