[Legate of Amon Lanc]

That's an interesting idea. A common reader, I believe, understands it the way that Celeborn simply thought that the Fellowship should not go through Moria - and which way they were supposed to take is another problem. But since the word needlessly is there, Celeborn's words cannot be reduced only on a negative sentence.

If you read the whole discussion with the Lord and the Lady, it gives (to me) the impression that C&G were expecting the Fellowship to come through Caradhras. There is this moment:

"Tell us now the full tale!" said Celeborn.
Then Aragorn recounted all that had happened upon the pass of Caradhras, and in the days that followed...

Aragorn starts with Caradhras, i.e. he starts with explaining why they could not pass Caradhras. This could mean that Celeborn knew that they were supposed to pass Caradhras, and Aragorn only explains why they did not. He is starting with Caradhras and does not think it necessary to include the reason why they passed Caradhras and not, for example, the High Pass or the Gladden Pass or even Gap of Rohan. He starts at the point to which he believes the others know the story. If he thought that C&G would have any questions on this like "Whoa, wait, and how did you get to Caradhras?", he would have started with explaining something else. It is, of course, possible that Aragorn (not being informed of everything) presumed C&G know the plan while they in fact did not (and they did not interrupt him because they considered it to be too impolite). But if Celeborn expected the Fellowship to go through Caradhras, looking at the word "needlesly", as you mentioned it, is in place. This suggests that although the preferred route, Caradhras, was closed, Celeborn knew of some B-route that was better than Moria. And this is indeed very strange. For if there was one, such as the Gladden Pass, and Gandalf or Aragorn knew about it, why did they not use it?
Aragorn says to Gandalf that they should try Moria only if other passages are closed:

Originally Posted by The Ring Goes South

But let us not speak of it again! Not yet. Say nothing to the others I beg, not until it is plain that there is no other way.

Then they simply return from Caradhras and go to Moria. At the beginning of the next chapter, most people speak the way that it seems that there is no way forward. Some of them don't know, of course, like the Hobbits, what paths are there, but still. Frodo:

Originally Posted by A Journey in the Dark

But how can I return without shame - unless there is indeed no other way, and we are already defeated?

And to this, Gandalf responds with the words that there is a way:

Originally Posted by A Journey in the Dark

"There is a way that we may attempt," said Gandalf. "I thought from the beginning, when first I considered this journey, that we should try it. But it is not a pleasant way, and I have not spoken of it to the Company before. Aragorn was against it, until the pass over the mountains had at least been tried."

Note the slight change of the sense of Aragorn's words. In the quote above, Aragorn says that they should not speak about Moria until it's clear that there is no other way. Here, Gandalf interpretates it that Aragorn wanted to try the Pass, but nothing more. I am not to say that Gandalf intentionally twisted Aragorn's words, but it's a thing worth mentioning. It simply seems that there is no other way. Why not? Gandalf himself says it:

Originally Posted by A Journey in the Dark

Since our open attempt on the mountain-pass our plight has become more desperate, I fear. I see now little hope, if we do not soon vanish from sight for a while, and cover our trail. Therefore I advise that we should go neither over the mountains, nor round them, but under them. That is a road at any rate that the Enemy will least expect us to take.

And when in next moment, they hear the wolves coming, he says:

Originally Posted by A Journey in the Dark

It is as I said. The hunt is up! Even if we live to see the dawn, who now will wish to journey south by night with the wild wolves on his trail?

It is as I said. So, the conclusion is: there could have been a B-path, the Gladden Pass for example, but obviously after the attempt to pass Caradhras Gandalf thought that it should not be used. It is possible that they did not agree with Aragorn on this, and that Gandalf (judging from his words) simply knew (presumed) in forward that once they try one of the passes, they couldn't use any other later. Simply: "Closing one pass for us means that the Enemy is/will be closing all of them the same/similar way if we try." This is logical and it connects the words of Aragorn with the way Gandalf interpretates them, though Aragorn maybe did not believe all the ways are closed until he saw the wolves. And even then, as we know, he did not like to go to Moria. And Celeborn's opinion could have very well been the same, from what we hear.