Are there any classical sources that criticize Avraham for coming very close to sacrificing his own son? Or do the mefarshim unanimously laud Avraham for his loyalty and faith? (I am aware of sources, such as Ralbag, that suggest Avraham misunderstood the commandment.)
I doubt you'll find any traditional sources that state he was at fault, given Hashem's express approval, the promise that it would be a merit - our constant merit - for all generations, and the importance placed on it throughout Jewish writings and tefillos. Also, until relatively recently, the idea of such sacrifice literally or figuratively, was a very real possibility. In a more figurative sense (and hopefully not literally), it still is. I wonder if there are even sources from the rishonim that felt the need to justify it (the reaction, I mean, not the blatant question of the command and retraction). It may have seemed poshut.
(I dont know if this is allowed here, I cna remove it if it isn't but are you looking for specifically criticism, or an understanding of how someone could do this and believe it's ok? That would be easier to find sources for)
As explained in the Gemara Taanit 4a, it is clear that G-d, blessed be He never intended to kill Yitzchok (ולא עלתה על לבי זה יצחק בן אברהם). The Gemara equates the episode of Akeidas Yitzchok together with the infamous episode of Yiftach sacrificing his daughter!