Well, the Orthodox view is that ecumenical councils are final and decisive.
That's obviously NOT the case, since we have had a need for councils to further explain and more narrowly define what previous councils meant. And that is natural. Unless we say that councils are entirely divine event with no human element (which would be Eutychianism) then we must agree that all councils have a human element, and it is of the nature of our humanity to be provisional.
"Now we see through a glass darkly"
Nicaea obviously didn't say all that needed to be said about Christ, because Ephesus 431 was necessary, and then Chalcedon had to say more, and so did Constantinople II etc etc.
I can't see why you have any problem recognising that, it is just a matter of fact.
Nicaea has complete authority as the pre-eminent creed of the Church, but it can be misunderstood and misused. One only has to look at liberal protestants who will say the creed but mean very different things to the Fathers. The Creed is only rightly understood within
the life of the Church. It does not stand alone, even the creed, as an external and decisive authority, it has authority IN the Church and within the rest of the Tradition.
How does the creed begin 'I believe in one God'. (Note that "I believe", yet you criticise me for using it). Ask a muslim if he agrees with this line? Of course he will. Does it mean the creed is defective, as you continually misinterpret any comment I make about any council, of course not. It means that the creed uses human language and does not communicate in itself the reality of what is spoken about, it is only a verbal symbol for something else, and symbols can be misapplied.
Look at suspenders. Over here that symbol stands for women's underwear, over in the US it stands for men's apparel. Both uses are legitimate but the mere insistence on using a word does not necessarily communicate.
The councils and creeds are authoritative IN the Church, interpreted BY the Church. That seems just a common-sense matter of fact. On their own, in a book read by a non-believer, or a liberal, or a Muslim, they may be taken as meaning a whole lot of things that the church never meant.
Therefore they are not a 'final and decisive authority' apart from the Life of Christ in His Church. Without the whole Tradition they can be taken as meaning anything, just like the Bible.
Do most Christians believe the Bible is an authority? Of course. Do all interpret it in accordance with the Fathers? of course not. Is this because the Bible is not a final and decisive authority? No, but it is an authority only when read with the Church, in the Church, by the Spirit of Christ. Anywhere else and it can be taken to mean anything.
Even these words will be misunderstood, taken to mean the opposite of what I am saying. But if you were here with me, if we had a wealth of other personal conversations to take into account which made clearer what I meant, then there would be no misunderstanding.
But anything human is of necessity and nature provisional and incomplete - as the scriptures teach 'Now we see as in a glass darkly'.
That doesn't mean complete confusion but it does mean a hesitancy to be dictatorial and polemical. It does mean that we must be careful we understand what the other is saying.
Nevertheless, for all of the provisionality of our earthly live, the substance of the faith is clear for all who live in the Church and are taught by the Church.
Again, I can't see the great theological issues which need a 'final and decisive authority' over the Church.