[Re Insights to Chulin 102:2,] I don't understand your answer to question 2.) on page 102 b below where you state that Rashi considers, for the purposes of receiving a punishment for eating an issur less than a k'zayis, only any entity which is either alive or was alive at one time as a biryah. Why can't we explain that there is no difference in the definition of a "biryah" for the purposes of bitul and for the purposes of receiving a punishment, and just as we see by a gid hanasheh that it is not botul in other gidim because it is a biryah even though it was never an entity that was alive, it is considered a biryah as far as the punishment as well.
As far as the question why doesn't Rashi choose to explain that the issur of "Ever min Hachai" applies to even less than a k'zayis because it is a biryah rather than explaining the chiddush that the Torah does not require a k'zayis of basar for this issur, we could explain that whether or not Ever min Hachai is considered a biryah would be dependent on a machlokes on the next amud whether or not an animal when it is born is considered destined to be cut up into individual limbs. According to the opinion that "L'evarim omedes" the issur of Ever min Hachai would devolve upon the animal at its birth and could be considered a biryah according to Rashi's definition that an entity is only considered a biryah relative to a specific issur if the issur was present at the creation of that entity.
However, according to the opinion that "Lav L'evarim omedes" the entity would not be a biryah in regards to the issur of Ever min Hachai since the issur was not present at the creation of the entity. In order to avoid an explanation that would fit with only one of these opinions, Rashi instead chooses an explanation for why Ever min Hachai applies to even less than a k'zayis of basar that will be agreeable to both opinions.
As far as I know, one never gets Malkus for a Biryah when that entity is not the "Shem Isur," like a Nemalah or a Gid. I do not think that one can get Malkus for a small Neveilah or a grape of Orlah, as the actual pieces being eaten are not the Shem Isur.
See Rambam Hilchos Ma'achalos Asuros 2:24.