This chiddush resolves R' Yosef Shaul Nathanson's question in his Divrei Shaul as to why the command of "V'nishmarta m'kol davar ra" appears in the context of war when it applies equally at all times and places. According to the Meshech Chochma, the pasuk's message is in fact one that applies specifically to battle.
Meshech Chochma adds that the Yerushalmi needs two pesukim for the issur of lashon ha'ra because the issur has two forms: There is the lashon ha'ra spoken amongst ourselves, "Lo teilech rachil," which is bad enough, but then there is the even worse lashon ha'ra of speaking about fellow Jews to others, the enemy or other outsiders, and creating a chilul Hashem. Airing Klal Yisrael's dirty laundry in public provides ammunition for our worst enemies.
I don't know why the M.C. doesn't quote it, but in light of this Yerushalmi I think we have a much better understanding of the Bavli's reading of the pasuk (Kesubos 5) in the same parsha, "V'yated ti'hiye lecha al azeynecha," as a hint that one's fingers should be placed in one's ears (a play on azeynecha / ozen) to avoid hearing gossip. The Bavli read the continuation of the parsha in a way that perfectly fits the theme of lashom ha'ra introduced earlier.
2. The Torah warns against having "eiphah v'eiphah," two different weights or scales. R' Shternbruch in his Ta'am VaDa'as homiletically interprets the pasuk as a warning against having two standards of behavior, one for the beis medrash, one for the outside. Perhaps the pasuk also means to tells us that we must use the same scale to weigh other's actions as we use to weigh our own. It's human nature to give oneself the benefit of the doubt, to take credit for success, to blame failure on other's lapses that got in the way, and of course to do just the opposite when evaluating the deeds of others. The Torah demands that we try to overcome that and evaluate others' deeds as we would our own.
3. Should we make anything of the fact that our parsha closes with a reminder never to forget Amalek just as we approach the 10th anniversary of 9/11? Who knows. In any case, aside from not forgetting that there is true evil out there out to destroy us, we should also remember the heroism of the NYPD and NYFD who are out there working to keep all us NYers safe.
4. Lastly, a bit of chizuk: The ben sorer u'moreh is nidon al shem sofo, he is punished severely because his gluttonous lifestyle will inevitably lead him to become a criminal. Rather than wait for the inevitable to unfold, the Torah assigns to him at the outset the punishment for the evil deeds that will follow. The Kozhiglover writes that midah tovah is greater than midah ra'ah. If a criminal can be punished for potential future acts because he already exhibits the desires that will lead him to that future life of crime, a person who exhibits the desire for kedusha and ruchniyus will be rewarded for attaining those values even if he has just begun to embark on the path to get there.