Is there a way to override and change a citation that Zotero creates, so that if I make a change to a specific citation once, it corrects that specific citation everywhere in my google doc?
What are you trying to change? If there's a problem with a particular citation, you should fix the data in Zotero. For other changes, you can customize the citation.
You should almost never make direct edits to citations in the document.
I am using Bluebook law review style-- it does not have an item type for court documents, for example. It also doesn't italicize case names, does not have a separate item type for regulations vs. statutes, etc. So I was wondering if there was an easy way to manually enter a citation and link it to a file without having to code the changes into the style.
You should almost never make direct edits to citations in the document.
(FWIW, Bluebook doesn't italicize case names)