Can n8n be used to authenticate with multiple different users?

The parameter authQueryParameters is set to access_type=offline&prompt=consent in the googleOAuth2Api and can so not be overwritten. But do not think you have to overwrite it, simply omit it.

Regarding the schema, there is an API endpoint for that:

That lists the available properties and as mentioned above, does oauthTokenData simply expect whatever the OAuth callback received.