The Ohr Sameiach and Rogatchover add a halachic dimension to the point. They quote the view of the Shi’ilitos who says that tnai kaful is only necessary when there is an agreement directly between party A and party B. However, where one of the parties is acting through a shliach, even without tnai kaful, any condition that is not met voids the shlichus and m’meila voids the agreement. At first the Bnei Reuvain and Gad thought that Moshe was acting as G-d’s agent, so to speak, and they therefore addressed Moshe as an independent actor, as “adoni Moshe.” However, when Moshe made a tnai kaful, they realized that Moshe was not simply an agent, because agreement with an agent doesn't require tnai kaful. It was as if Shechina m’daberes m’toch grono, G-d was speaking through Moshe, and their agreement was being made directly with G-d. Therefore, they rephrased their consent to “es asher dibeir Hashem.”
Technical details aside, Ralbag sees a moral lesson in the fact that Moshe framed the agreement in the formal terms of tnai kaful and did not just seal the deal with a handshake. Unless terms and conditions are spelled out up front, parties can wind up disagreeing later as to what was meant; each side may think the other is in the wrong – even if that other side being questioned is Moshe Rabeinu. Sure, we would give him the benefit if the doubt, but the greatness of Moshe is that he leaves no doubt.
2) Rashi writes that sheivet Levi participated in the war against Midyan. The GR”A, however, had a different girsa in the Sifri and opines that Levi did not participate. You could try to reconcile the two positions by saying they did not participate directly in battle but still contributed to the war effort. We once discussed the Rogatchover’s sevara that the battle against Midyan was an act of nekamah and did not fall into the formal halachic category of milachama, with all is various rules. This may be the point of the machlokes. Sheivet Levi did not participate in milchama (Rambam, end of Hil Shemita), but this may not have been a milchama.
3)The Tosefta writes that the “klei kodesh” taken out to battle was either the aron or the bigdei kehunah. The Netziv back in Parshas Beha’aloshecha (10:9) in the parsha of chatzotzros writes that the pasuk there of “v’nizkarten lifnei Hashem Elokeichem” teaches that the chatzotzros may only be blown in the presence of the aron or the tzitz (which had the shem Hashem on it). This perhaps explains the view of the Ba’al haMaor who writes that blowing chatzotzros on a ta’anis, as opposed to on Rosh haShana, is only a din derabbanan. What about the pasuk in chumash that says there is a mitzvah to blow at a time of tzarah? It could be that the Ba’al haMaor is talking about blowing outside the mikdash, outside the presence of the aron. The mitzvah d’oraysa, as the Netziv explains, is only in the mikdash when the aron is present. (R' Soloveitchik quoted here suggested that the Ba'al haMaor of course holds that tekiyas chatzotzros is d'oraysa; what he meant is that you cannot say mitzvos lav lehenos nitnu on the mitzvah of chatzotzros because it is a chovas hatzibur, not a chiyuv on each individual to blow).
4) If anyone does not know about the Shmira Project, please check out their website http://shmiraproject.com/ It's wonderful to say tehillim or learn or do mitzvos in the zechus of our soldiers in general, but the shmira project goes a step further and will pair you with a specific person in whose zechus you can learn, daven, do mitzvos.