Before I try to answer your questions, allow me first to expand on it. The Gemara (at the top of the page) makes it clear that one does not transgress "Lo Yavo" until most of his body enters the Ohel ha'Mes, front-first (see also Tosfos and Rosh). Obviously, the Gemara understands that "Lo Yavo" means more than just "do not become Tamei through Ohel ha'Mes." It means "do not come into an Ohel ha'Mes." If so, how can the Gemara now suggest that simply by sitting in an Ohel when a Goses dies, he has "come into" an Ohel ha'Mes and transgresses "Lo Yavo?"

It could be that the verse is to be read, "Do not enter a house in which a dead person either is, or will be ." If this is true, then by entering the house of a Goses, the Nazir is liable to transgress the Isur if the person dies while the Nazir is still standing in the house. The Hasra'ah for the Isur, then, would have to be before the Nazir enters, and not just before the Goses dies.

(This would of course raise the problem that the Hasra'ah is a Hasra'as Safek, since we cannot be sure that the Goses will die while the Nazir is still there. We cannot warn the Nazir that he will definitely transgress the Lav when we see him entering the house. However, the question from Hasra'as Safek remains regardless of what we suggest. Even if the Hasra'ah is given after entering the house and before the Goses dies, it is a Hasra'as Safek since the Isur depends on an outside action (the death of the Goses) that is not sure to occur. The Shitah Mekubetzes asks this question on the Sugya -- how can we give Malkus if it is a Hasra'as Safek? The Rishonim answer either that the Sugya holds Hasra'as Safek Shmah Hasra'ah, or that since most Gosesim die it is considered a Hasra'as Vadai; see Rosh.)

However, the Rishonim (see ROSH in Shitah Mekubetzes) write that the Gemara could just as well have suggested that the Nazir transgresses "Lo Yavo" if he is sitting in an Ohel when another person brings a k'Zayis of Mes into the Ohel. From this it is obvious that they consider the very act of being with a Mes in the Ohel a transgression of "Lo Yavo." It is not necessary to "enter" the house in the usual manner.

It seems that their proof for this assertion is from Rav Papa's statement, that if a Nazir enters a Beis ha'Kevaros while inside a Shidah, and his friend removes the floor of the Shidah, he transgresses "Lo Yavo," even though he did not do an act of entering in the normal manner (since he was inside the Shidah when he entered -- see CHAZON ISH 143:106, towards the end, cited in ARZEI HA'LEVANON #307, who wondered about this point).

The answer to your question, though, can be learned from the Gemara in Shevu'os 17b, which says that Bayis Menuga is only Metamei with "Bi'ah," entering in the normal fashion, and therefore if a person enters it backwards he is not Tamei, even if only his nose remains outside the house. However, when his entire body (i.e. even his nose) enters the house backwards, he does become Tamei. The Gemara asks why that is, and the answer, basically, is that it is not logical to suggest that the Pasuk wants him to remain Tahor even when he is entirely inside the Bayis Menuga. Accordingly, the same answer may be given here: it is not logical that the Pasuk wanted him to be permitted to be fully inside the Ohel ha'Mes, if even entering the Ohel (k'Darko) is prohibited.

This, then, explains why the person in the Shidah transgresses "Lo Yavo," and why the person with the Goses transgresses "Lo Yavo."

Mordecai Kornfeld