"Yet" is an adverb. Adverbs typically precede the words they modify. And yet...
It's not a split infinitive, which is "to [adverb] [verb]"; nothing ever goes between "to" and a verb. But this is similar. Probably a rule, but I don't know what it's called.
You must say "I have not been"; you can't say "I not have been". But you can say "I never have been", "I always have been", etc. But most people say "I have never been", "I have always been", etc. In fact, putting "never" and "always" between "have" and "been" is far more common.
It seems awkward to say "No agreement yet has been reached." But maybe that's just because your alternative, "No agreement has been reached yet" is so common. And I'd say "yet" at the end is far more common than "yet" between "has" and "been".
That's why "always" and "never" (can? should?) go before "has been". But, as I said, that's just not the case (or doesn't seem to be) with "yet."
In many (most?) cases, the position of the adverb affects the meaning.
Only Jim ate the cake. (No one else ate cake.)
Jim only ate the cake. (Jim didn't do anything else with the cake.)
Jim ate only the cake. (Jim didn't eat anything other than the cake.)
Jim ate the cake only. (Nope.)
Always Jim has been a good student. (Nope.)
Jim always has been a good student. (Yes.)
Jim has always been a good student. (Maybe.)
Jim has been always a good student. (Nope.)
Jim has been a good student always. (Nope.)
Yet Jim hasn't been a good student. (Nope.)
Jim yet hasn't been a good student. (Nope.)
Jim hasn't yet been a good student. (Maybe.)
Jim hasn't been yet a good student. (Nope.)
Jim hasn't been a good student yet. (Yes.)
"Yet" must be one of them irregular adverbs.
This phrase comes from a very old superstition that naming the Devil would cause him to appear — see The Phrase Finder.
Over the centuries it has developed into a light-hearted saying that doesn't imply hatred of the person arriving, though it would depend on your relationship with them (and the tone in which you said it) whether you thought it was appropriate. I seem to remember my own grandmother, many years ago, saying "Talking of angels..." instead, but I can't find any reference online to this or similar expressions, so maybe it was her own invention.
Best Answer
It's a very subtle difference. "Could not have" is used to definitively declare that an impossibility occurred in the past. As in, it was impossible for the person to be reached. It assumes the writer has full knowledge of the possibility of an activity. "Could not be" expresses that an attempt was made by the writer to reach the person, but it was unsuccessful.
An example is: "He could not have taken a call at 5pm because he was on a flight to Baltimore." The person declaring that would have to have that knowledge to know the impossibility. Another person who didn't have such knowledge would say, "I attempted to call him, but he could not be reached." They don't know why he couldn't be reached, or that it was impossible to do so.