The Netziv writes elsewhere as a general rule that any chiddush that changes the status quo must be lishma (see post here). Doing an aviera, even for a good purpose, is certainly a great chiddush; therefore I would suggest that it falls under the same general requirement of lishma that applies to any chiddush.
We also one discussed a yesod from R’ Amiel, who explains that the reason lishma is so critical by korban pesach, even more so than korban chatas, is because pesach celebrates the founding of Klal Yisrael. When one is starting a new enterprise it is absolutely essential to get every detail right, because if the foundation is lacking, the rest of the building won’t stand. (It’s already Kislev and Chanukah is coming – R’ Yosef Engel writes that even though tumah hutra b’tzibur, the menorah had to be lit with tahor oil because the first lighting, the act of dedicating the menorah for use, has to be done perfectly). Yitzchak’s bracha was not just about getting “tal hashamayim u’shamanei ha’aretz,” material good, but it was about choosing who would carry on the legacy of the Avos and build Klal Yisrael. It was the foundation, and therefore had to be done with the purest intentions.
2) I can’t find the Sefas Emes – Likutim online, but would love to link to it because I think you have to see this one inside. What’s the idea behind the malachim crying in Yitzchak’s eyes to blind him? The Sefas Emes highlights the pervious lines in that same Midrash (63:10) that teaches that someone who has a talmid or a son who is wicked is stricken blind. It seems from the Midrash that this is for the person’s own sake, so that he should not venture outside his home and be the subject of gossip, e.g. “There goes so-and-so whose son is the troublemaker.” The S.A., however, reads it as a punishment as well. What the Midrash is telling us is that it’s not just blindness that Yitzchak suffered because he had an Eisav and loved him, but it goes back further -- even the trial of the akeidah was due to this reason! It’s quite a chiddush (I hope I am not misreading his meaning. If anyone takes a look and interprets what he says differently, pls comment!)