All night or just until midnight?

The Mishna in Brachos daf 2a says that any time the Chachamim said that you have until Chatzos, you really have all night but the Rabonon just made a takana that it should only be until midnight. The simple reading of the Mishna and the way that the Rambam reads it is that it then lists two other examples - The hekter chalavim v'eivarim and the eating of korbanos which can only be eaten for one day and one night. Those two mitzvos together with krias shma are mitzvos that according to the Torah last all night but the Chachomim said you must finish by midnight (see Rabeinu Yona for a discussion if the Chachomim said this l'chatchila or even b'dieved so if you missed krias shma by midnight then maybe you shouldn't say it at all).

Rashi disagrees with the Rambam and explains that the three mitzvos listed in the Mishna are just examples of mitzvos that last all night but the gezeira of the Chachomim to finish by chatzos was only said by eating the korbanos and by Shma but there was no such gezeira made with regard to hekter chalavim.

A couple of questions bothered me with Rashi's explanation.
1. This Mishna implies that the takana was made for all three mitzvos so what compelled Rashi to explain otherwise?
2. Why was the takana made only by Shma and Korbanos but not by hekter chalavim?

The Meforshim on the mishnayos attempt to answer the first question. The gemara in no other place mentions this takana by the hekter chalavim so it must be that there was no such takana so because there's another way to explain this Mishna, Rashi chose that explanation.

However, the question remains why was there no takana made by hekter chalavim. The truth is that there is another strange thing in Rashi. When he explains the reason for the takana, he gives two different explanations for Shma and Korbanos. He says that by shma they made the takana to be mezareiz people to do the mitzva so they shouldn't procrastinate and by korbanos he said that they made the takana so people shouldn't eat into the morning at which time it will be nosar and there will be a chiyuv kares. Why doesn't Rashi just give the ziruz explanation for both mitzvos? It could be that the reason the ziruz explanation doesn't work for korbanos is because most of those are kodshim kodshim (most kodshim kalim could be eating the second day also so aren't included in our Mishna) which can only be eaten by kohanim and kohanim zerizim heim. So the Chachamim would not have made a takana to be mezareiz the Kohanim. They did make the takana though because the eating is a continual process so it's possible that they will start at the correct time but it will continue until the morning at which time the chiyuv kares will be upon them. This will then answer our orginal question - there was no need to make a takana for hekter chalavim. First of all, we're dealing with Kohanim so there the zariz reason doesn't apply. Second of all, it's not a process of eating that continues. They just drop them on the mizbeach and they're done so we don't have the fear of it continuing into the morning.

UPDATE (9:40 PM): R' Feldman from YU has another answer to this question (much better than mine).