There are two different types keys for " Service account credentials " JSON and P12
Please, which one is indicated to work with "Google MC Funnels API"?
Either can be used with MFC API. Service account credentials and Oauth2 credentials have nothing to do with the which API you can access. Except with regard to some APIs which don't support service accounts (YouTube API , and blogger to name two). Service account and Oauth2 are all about gaining access to the data you need.
Service account p12 file
Service accounts are used to access data you the developer have access to. If you want to grant someone else access to see your Google Analyitcs data you would use a service account. Setup will require that you take the service account email address and add it as a user at the account level in the google analytics website admin section. it must be the account level.
Oauth authentication Json file
Oauth2 will request a user give your application permission to access their data.