I know that the following was not your question, albeit one for which you proposed an answer. I simply wish to propose a different answer than yours ...Are you aware of any single "authority" which speaks for all Protestants?Yes, ... JESUS speaks for all true Christians.
To this effect, He said ...
Matthew 12:30 He that is not with me is against me; and he that gathereth not with me scattereth abroad.Luke 11:23 He that is not with me is against me: and he that gathereth not with me scattereth.
... so that it appears that, to be a bonafide part of his church (i.e. his disciples, his followers), one must be ... with Him.
On the other hand, JESUS also says ...
Mark 9:38 And John answered him, saying, Master, we saw one casting out devils in thy name, and he followeth not us: and we forbad him, because he followeth not us.39 But Jesus said, Forbid him not: for there is no man which shall do a miracle in my name, that can lightly speak evil of me.
40 For he that is not against us is on our part.
41 For whosoever shall give you a cup of water to drink in my name, because ye belong to Christ, verily I say unto you, he shall not lose his reward.
Luke 9:49 And John answered and said, Master, we saw one casting out devils in thy name; and we forbad him, because he followeth not with us.
50 And Jesus said unto him, Forbid him not: for he that is not against us is for us.
... indicating that we need not be overly concerned with abiding under the same tent. Our unity derives from our being with JESUS.
<> Interesting point of view, for one who is following Jesus. I guess He was just joking in Matt16:18.<>