1) The Shulchan Aruch (O.C. 58:1) Paskens, based on our Mishnah, that one can recite Kriyas Shema in the morning from the time when he is able to recognize a good friend from four Amos away, until three hours (Zemaniyos) into the day. b'Dieved, one can fulfill Kriyas Shema if he recites it anytime after Alos Hashachar. The Mishnah Berurah (58:2) explains that as some people rise at that hour, there is still a fulfillment (b'Dieved) of the Pasuk "u'Vekumecha."

2,4) The Shulchan Aruch (89:1) Paskens, based on the Mishnah and Gemara in Berachos (26a), that the optimal time for the Amidah of Shacharis is ha'Neitz ha'Chamah. One can Daven, as is accepted practice, through the fourth hour of the day (Zemaniyos). b'Dieved, one can still Daven Shemoneh Esreh until Chatzos, and as early as Alos ha'Shachar.

3) The Shulchan Aruch (58:1-2) Paskens that it is optimal to read Kriyas Shema before ha'Netz ha'Chamah, as do those who Daven Vasikin. It is therefore clear that there is nothing wrong with solely reading Kriyas Shema before ha'Netz ha'Chamah, unless it is before the time he can recite Kriyas Shema l'Chatchila (as stated above in answer #1).

5) Not that I am aware of.

Note: I assume that the questions above were asked in the context of Halachah l'Ma'aseh, not whether or not there are opinions, which are not Halachically significant, which maintain otherwise.

All the best,

Yaakov Montrose