The Mishnah Berurah (OC 63, note 16) summarizes the first half of your question by saying that there are three primary views among the Rishonim about what part of Kri'as Shema is mid'Oraisa: 1) the Rashba and the Beis Yosef: only the first Pasuk, 2) Rashi: the whole first Parshah, 3) the Rambam: the first two Parshiyos.

The Pri Chadash (OC 69) answers the second half of your question by bringing strong textual proofs for each of these opinions (and in the end concludes that the third opinion is the strongest). (He also argues that the third Parshah cannot be considered part of the Torah requirement of Kri'as Shema even though it is a Torah Mitzvah to recite it in order to remember Yetzi'as Mitzayim.)

The Kiryas Sefer (on the Mishnah Torah, Hilchos Kri'as Shema) says that even though one has fulfilled his Torah obligation after reading the first Pasuk, he can extend the Mitzvah to include all three Parshiyos by continuing to have the Kavanah of Kabalas Ol Malchus Shamayim.

Kol Tuv,

Yonasan Sigler

This is not a Psak Halachah

Yes. Tosfos (Sotah 32a) seems to suggest that the entire Mitzvah is only Rabbinical. This is consistent with the Gemara (Berachos 21) that says that if one is in doubt whether he recited Shema, he is not required to say it again (see Rabbeinu Yonah, op cit.).

Y. Sigler