ואם תאמר אם שמע כן מפי הקב''ה מה מקום להקפדתו? אולי לצד שלא אמרו עדיין לאהרן, חש שדן בו טעם אחר שאינו צודק ושרפו
The Ohr haChaim here has an amazing chiddush. He writes that Moshe knew there was a chiluk between kodshei sha’ah and kodshei doros and that’s why one of the three korbanos was burnt and the others eaten (parenthetically, the Targum Yonasan writes that all three were burnt!) Moshe had heard this chiluk directly from Hashem. So why was Moshe upset at what Aharon had done? O.C. answers that Moshe had yet to tell Aharon this chiddush din, and therefore, he had no idea why Aharon had distinguished between the korbanos. Was it because Aharon had intuited the chiluk Moshe had learned, or was it because of some other sevara?
The sefer Nesivei Chaim points out that it seems from here that just fulfilling the mitzvah correctly was not sufficient -– Aharon had to get the “why,” the sevara, right as well.
(It could be, as he suggests, that this was only true because Aharon had not been given these dinim as a mitzvah yet. Once something is given as a mitzvah, just doing the correct actions even without the correct sevara is enough.)
2) The simple pshat in “vayitav b’einav” is that Moshe was satisfied with Aharon’s response and glad that no wrong had been committed, i.e. he breathed a sigh of relief. The Seforno, however, says it means much more than that. He explains that Moshe was happy to hear such great lomdus from his family; “vayitav b’einav” that his brother and nephews were such talmidei chachamim. Divrei Torah are always m’samchei lev, but it’s especially m’samchei lev if it’s your brother, nephew, or especially son or daughter who is the one saying over that Torah.