I don't agree with either of those statements. I don't know which versions you read, but in the French translations and original Polish it is clear to me that Ciri explicitly brings this question up to sow doubt into the reader's mind as to what actually had happened to Auberon and what Eredin had planned/was expecting from it.
Exactly. But it's important how Eredin reacts to Ciri's question or better statement. He is actually surprised by the information that Auberon is dead which clearly indicates that he didn't kill him. I do agree that we don't really know what he means when he asks Ciri to give Auberon the phial. But that's of course before Auberon's death and the scene in which Ciri confronts Eredin. The suicide story is also fitting to how the whole scene was built up.
Let's have a look. First the scene with Ciri and Eredin in which he tries to convince her to give Auberon the phial:
"Surely", [Eredin] slowly began, "you've alreaedy managed to get to know Auberon a bit. Surely you've already mentioned how incredibly ambitious he is. There are things that he will never accept, things that he will never even acknowledge. He rather dies."
Ciri kept silent, bite her lips and quinted towards the cot.
"Auberon Muircetach," the elf continued, "won't ever use magic or other means to change the current situation. But there are such means. Good, strong, reliable means. Much more effective than the attractors with which Avallac'h's servants accumulate your cosmetics."
This whole passage (I only used a small excerpt here) has two dimensions. First, what is said? Eredin says that Auberon would rather die than acknowlediging that he isn't capable of impregnating Ciri. And he wants Ciri to give him a phial with a liquid that should enable Auberon to do so, at least that's what he's implying. Is that the truth? Well, we don't know and it's left open in that scene. But there is a second dimension. We see this whole scene mostly through Ciri's eyes and she is very prejudiced towards Eredin. With her (extremely bad) experiences with people she assumes that others always want to abuse her and that everybody always wants to achieve the very worst. So she naturally thinks that Eredin wants to kill Auberon instead of "helping" him with his little potency problem. So the reader indeed is led on this way of thinking, by both Ciri's perspective and the open ending in this scene.
Let's have a look at the scene in which the dying Auberon talks with Ciri:
"Despite all of that", [Auberon] said, "I resent dying. And I feel incredibly sorry that I have to. Who whould have thought. I have thought that I wouldn't feel sorry. I've lived a long live, enjoyed everything to the full. Now I feel only tedium for everything. But nevertheless I feel sorrow now. And you know, what else? Bow down to me. I will speak it in your ear. I shall be our secret."
She bowed down to him.
"I'm scared." he whispered.
"I know."
"Are you with me?"
"Yes."
"Va faill, luned."
"Goodbye, erlking."
So what can be said about this scene. If you ask me Auberon doesn't show the behaviour of somebody who was killed by somebody else. He is calm, not angry. He also says that he only feels tedium for everything which implies that he doesn't have much will to live much longer. He also says that the thought that he wouldn't feel sorry which again implies that he maybe committed suicide.
But then again there is the phial with the green liquid inside which looks very much like the same phial Eredin wanted to give Ciri. So it's quite natural that Ciri assumed that Eredin succeeded with his plan to kill Auberon. She already though so from the beginning and this is the perfect proof for her.
So, final look on the third scene in which Ciri confronts Eredin:
"We return, Zirael. Auberon is waiting. Tonight, he will be strong and full of zest for action, I vouch for it."
"My ass", she repeated. "He has drunk to much of that invigoration agent. The one you gave to him. But perhaps that wasn't an invigoration agent after all?"
"What are you spaking of?"
"He is dead."
He immediately recovered from the surprise, suddenly lunged at her while he caused the boat to totter.
To me it seems pretty obvious in that scene that he doesn't know about Auberon's death. While he could have lied to Ciri before he was actually surprised when she told him. You can't be surprised if you've done it yourself.
So what could possibly have happened? My personal guess is that Eredin really wanted Auberon to impregnate Ciri. Therefore he gave the king the phial. But the king is too proud to use such an agent and exchanges the agent for a deadly one, rather killing himself than acknowledging that he isn't able himself to impregnate Ciri, like Eredin himself told Ciri before. If he told Ciri the truth about Auberon the king wouldn't accept such an agent. And why should Eredin lie about Auberon here? He has no reason to lie about that if you ask me. Or maybe it was indeed an invigoration agent and it just killed Auberon, like an accident. That doesn't seem very likely imo, based on the vast knowledge the Aen Elle have but it's still possible. In that case Eredin might have some responsibility for Auberon's death, but he neither murdered nor killed him. It was Auberon who drank the agent voluntarily, with nobody forcing him to do so.
(I've read the German version by the way which is said to be actually very closely translated to the Polish version so that shouldn't be a big issue here. If that's not true please tell me.)
I think this chapter of the book is probably one of the weakest in the series but Eredin steals the show from Avallac every time. That the author can suggest two very different behaviours of Eredin and both be seen to be plausible actions taken by his character, I think speaks to how realized he is as a character. Avallac on the other hand, is hardly what I would call someone with an extraordinary amount of depth. His development is both blandly straightforward and not nearly as interesting or subtle as Eredin.
Which two different behaviours of Eredin do you speak of. I can't see them tbh. And subtlety? Maybe mystery, but I wouldn't call it subtlety. And Avallac'h on the other hand is more complex because he's personally involved in the whole story, not only politically (at least based on what we know). He onced loved Lara Dorren. And his real motives and agendas are even more hidden than Eredin's and not straightforward at all. Sure, he tries to convince Ciri to get a child with Auberon. Eredin wants the very same thing, by the way. But we don't know how far Avallac'h would go to get what he wants. Eredin suggests that he "might use his lab if Ciri doesn't want to give birth to a child with Auberon". So maybe he'd do similar things Vilgefortz had planned for Ciri? But then again, could he actually do it, always seeing Lara in Ciri's eyes? You see Avallac'h has at least as many question marks above his head than Eredin. Maybe you like Eredin more or you find him more fascination, but that doesn't make him more complex.
And on a final note, I find this chapter being one of the better ones of the books. So the prespectives and opinions can differ I guess.
