It seems Salesforce doesn't allow me to create a custom label for this field.
Aside from overriding the Opportunity's page layouts with custom visualforce pages, what are my other options?
My current employee has renamed Accounts to Companies and this field now proudly states "Company Name". I don't know how they did it (I wasn't around) but I suspect it happens automatically when you rename Accounts tab? Go to Setup -> "Rename Tabs and Labels" and play with it?
Alternative would be for example to switch whole org from "English" to "ENglish (UK)" or other way around and translate the field ;)
I'm not entirely sure what you're asking for here.
Opportunity.AccountID is a referential field, but account ID is the SFDC 15-character identifier for an account (starts with 001).
Are you wanting to rename the field in the UI? For reports?
What's the end goal, that will make it easier to help you out.