Regarding the prohibition of eating before Shacharis, when does this prohibition start? If one wakes up before Alos HaShachar would one be able to eat or drink.
The prohibition of eating before Shacharis only begins after Alos Ha-Shachar, when it is possible to daven.
According to Talmidei Rabbeinu Yonah (5a), one who begins to eat before Alos must stop when Alos is reached, but the Rashba rules that one does not need to stop. The Shulchan Aruch (89:5) rules that one has to stop when Alos arrives, but mentions the lenient opinion. The Mishnah Berurah (29) rules like the stringent opinion.