How do we answer Catholics and Orthodox on authority?

[This is to ask the reader for help in answering this common line of argument from the other two branches of Christianity against the Protestant branch:]

Sola Scriptura means: Scripture alone. It is one of the five solas of the reformation. It means that Scripture is the final authority on matters of truth. Yet, in regards to Scripture, there are two levels:

Revelation: What God says
and
Doctrine/Interpretation: What we say revelation means.

So "Sola Scriptura" says nothing about who has the right doctrine/interpretation. Here's a test case:

Does baptism save?

What does Scripture say?: "baptism now saves you" 1 Peter 3:21.

Thus, when speaking about this passage, we have no disagreement over what Scripture says, Scripture says "baptism now saves you," even mentioning the water in particular. We do have a disagreement over what Scripture means.

But here is the question: Who decides what it means? Catholics appeal to the See of Peter as authoritative interpreter, Orthodox appeal to the Seven Ecumenical Councils as the collective interpretation of the Church, Protestants have:

1. The clear testimony and reading of Scripture? [the clear reading is "baptism saves"...]

2. The internal testimony of the Spirit? Whose internal testimony? Mine? My Protestant Lutheran brother who quotes the Augsburg Confession that says "Baptism they teach that it is necessary to salvation" [link]? Or my Protestant Reformed brother that quotes the Westminster Confession that says "salvation [is] not so inseparably annexed unto [baptism], as that no person can be regenerated, or saved, without it" [link]? [both the Lutheran and Reformed brother claims Sola Scriptura]

3. Human Reason? That same "reason" that Luther called a, ahem, prostitute? Again, whose reason? Mine? Yours? If they contradict, who decides? If reason, why do we need revelation which is "foolishness" to those reasonable Greeks?

My answer: I have none, that's why I am throwing it out there. I do not believe water baptism saves [I side with the Reformed brother in question 2, as I believe most people reading this blog do] just as I believe justification is by faith alone though the "plain words" of James say differently. But how do I as a Protestant escape taking the Pope's hat off the Bishop of Rome, and putting it on my head? How do I avoid making my self and my personal interpretation the only council I listen to? Or do I have no such assurance?

[This post is not meant to imply Catholics and Orthodox have no problems here as Orthodox have no means of addressing new theological questions after the Seven Councils and the fact that the doctrine of Papal Supremecy did not develop until the 5th Century and many would say their answers have contradicted each other. I'm merely concerned with the Protestant problem here.]