I have heard these and similar things from the Dimonds as well.
But considering that after the Council of Trent the Pope established a seperate congregation specifically tasked with interpreting the council's decrees, the idea that interpreting a dogma is not allowed must be wrong.

Ridiculous. Arguably the greatest of all the councils in the Church purposely did not teach clearly? Are we to believe that Trent taught infallible ambiguity? Balderdash.

"Hence, too, that meaning of the sacred dogmas is ever to be maintained which has once been declared by holy mother church, and there must never be any abandonment of this sense under the pretext or in the name of a more profound understanding." - First Vatican Council

Ridiculous. Arguably the greatest of all the councils in the Church purposely did not teach clearly? Are we to believe that Trent taught infallible ambiguity? Balderdash.

"Hence, too, that meaning of the sacred dogmas is ever to be maintained which has once been declared by holy mother church, and there must never be any abandonment of this sense under the pretext or in the name of a more profound understanding." - First Vatican Council

It says the the understanding, meaning, and sense must be retained.

It says the the understanding, meaning, and sense must be retained.

Yes of course, which is to say that there is every reason for a Pope to condemn "a separate congregation specifically tasked with interpreting the council's decrees" were such a thing to exist, not establish one.

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Is it? Seems every reason to have authoritative ways of interpreting conciliar teaching.

If it were your Council and after 18 years of it, would you send the whole mess out to a separate group so they can figure out what it is you actually said?

Intuition doesn't seem like the right rule by which to judge.
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A better question is: if the Church intends conciliar decrees to only ever have a strict literal sense ('the way the words read once and for all', as it is often affectionately put), why then, when given the opportunity to explain how she must be understood (the Vatican I passage you quoted), does she not say so?

But she does say so, she says they are to be understood "as once declared," not understood the way "a separate congregation interprets them."  

It says the meaning once declared is to be maintained. It does not say that the literal or plain meaning of a given declaration is to be understood literally.

Let me rephrase: Vatican I does not say that the plain/literal meaning of a given conciliar decree is in fact the correct or complete meaning of a given conciliar decree. 
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It simply says that whatever meaning the Church declares is the meaning to be retained. 

It says the meaning once declared is to be maintained. It does not say that the literal or plain meaning of a given declaration is to be understood literally.

Well, if the meaning of that which is declared is to be maintained as declared, but can be understood in any other way at all, then it seems obvious that it's meaning will not be maintained. Heck, that's pretty much the same formula that V2 went with, ie say one thing but can mean some thing(s) else.

Clear teachings get misunderstood due to people misunderstanding what is being taught, not because what is being taught is not clear - preconceived notions might be the biggest culprit. By that I mean people take their mindset to these teachings and they see in these various teachings what they already believe. And what they do not believe, they do not see, or refuse to see, not sure which.

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I see. So there's no breaking news here, just the satisfaction of your un-evidenced prejudice.
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No need to be rude. I was just happy that Mr. Derksen hadn't fallen into this heresy.