Are there any Mefarshim who commented on why Avraham chose to advocate on behalf of Sodom when G-d tells him that the city will be destroyed, but chose to remain silent and just accept the decree when he was told to sacrifice his son?
I hope you'll accept my memory lapse as to who said it, but I once saw one of the mefarshim say that by Sedom, Avraham was "tipped" as to the fact that he should advocate, as he figured "why else would G-d be telling me if not that I am supposed to do something about it?" (Similar as to how Moshe knew to "argue" with Hashem about destroying the Jewish people after the cheit ha'egel.) But by the akeida, there was no such message, because he was the key player involved and there was no reason he shouldn't have been told.
On another note, someone once pointed out to me that in Selichos, one of the aneinus is addressed to "He Who answered Avraham on Har HaMoriah" - the only time we know of that Avraham went to Har HaMoriah was for the Akeida, and a teacher of mine suggested that he was being answered on a request to spare Yitzchok.
My rav explained: (IY"H, when I see him next week, I will ask him where he got this source, and edit it.) For now, the explanation sounds credible. Sorry - I don't want to reveal his name...
Child sacrifice was common and customary at that time. Therefore, Avraham did not protest the request. Yes, Avraham probably was pained that he would have to sacrifice his own son, but, as he was devoted to G-d's commandment, he surmised that since this was something that everyone else did, it wasn't unusual of G-d to request this of him.
As a matter of fact, this was part of the "test". Hashem tells him at the end, Breishit 22:12 , "NOW I know that you are a G-d fearing person...". Didn't G-d (the angel) know all this before? But, it was to prove that BECAUSE Avraham was G-d fearing, this was the reason that Avraham sacrificed his son, and not because everyone else did it and he was following local custom. In other words, by doing so, Avraham made clear to everyone in the future that Jews DON'T sacrifice their children.
This is a good question, but some more facts need to be taken into consideration:
Avraham sacrificed himself in אור כשדים jumping into the fire, we need a source that explains why sacrificing his son is any harder.
Itzhok was 37 and lived with his parents. Maybe Avrohom didn't love him "so much" after all and needed to push him out? As you can see, I don't accept anything for granted.
We don't know what exactly G-d said to Avraham that made him comply so fast. Tora's description is very scarce only mentioning "וְהַעֲלֵהוּ שָׁם לְעֹלָה" - no mentioning of a bloodshed at all. It was Avrohom's idea to cut the boy (the grownup, sorry).
Obeying to a prophecy is a Mitzvah, and Avrohom did it every single time - leaving Ur Kasdim, circumcising himself and his family, sacrificing his son and more. This behavior is consistent with Avrohom, so the only question left should be on arguing about Sodom, not about the Akeidah.
The Gemmorah in Sanhedrin 89b states that Avrohom did argue with G-d about the Sacrifice if you like:
את בנך ב' בנים יש לי: את יחידך זה יחיד לאמו וזה יחיד לאמו אשר אהבת תרוייהו רחימנא להו את יצחק וכל כך למה כדי שלא תטרף דעתו עליו
God said to Abraham: “Please take your son, your only, whom you love, Isaac” (Genesis 22:2). When God said: “Your son,” Abraham said: I have two sons. When God said: “Your only,” Abraham said: This son is an only son to his mother, and that son is an only son to his mother. When God said: “Whom you love,” Abraham said: I love both of them. Then God said: “Isaac.”
Now to the "real" answer:
According to Ariz"L (don't remember exactly where) here's an explanation in simple English:
Itzhok was 37 and he was "stuck" in life (as he was all Gevurah/feminine etc. That's why he looked so different from Avraham that others started to question his fatherhood. As Avrohom was Chessed). He was so Gevurah he couldn't procreate (we already know that) and therefore make the continuation of the Jewish nation G-d promised to Avrohom.
In his prophecy, G-d clearly explained to Avraham that the only way to change the situation was by making a "serious remake" to Itzhok's soul by an extraordinary event of taking his soul out (for the treatment). So he will become a "man" and procreate.
The plan worked out so well G-d needed to stop Avrohom from actually hurting his son because Itzhok played along and his soul flew away before the damage.
G-d made necessary adjustments to his soul, taking his feminine part out and making Rivkah's soul (don't ask me how).
I don't understand still why it was so hard for Avrohom to explain this to Sarah.