1. According to Rabbi Yehudah that plag is night (based on fact that you cannot daven mincha anymore and one can recite kiddush), why can't you recite krias shema, say sefira or eat terumah then - kohanim?
2. If plag is night and that's when you can be makabel Shabbos, why is it up to the individual to decide what precise time he will be mekabel; shouldn't it automatically be at plag?
3. How is there a bein hashemoshos according to Rabbi Yehudah?
4. What is one liable for if he does melocha after he is mekabel shabbos before shekiah?
thank you
1-3. The Chidushim U'Biurim in Berachos (27a) clearly states that Rebbi Yehudah does not hold that night starts at Plag ha'Minchah. One proof he cites is the fact that the Gemara thought it possible that Rebbi Yehudah was referring to Minchah Gedolah, which is only 6 1/2 hours into the day and clearly not nighttime. He explains that Rebbi Yehudah is merely stating a time when Zman Minchah ends, and hence Zman Ma'ariv starts. This is because Rebbi Yehudah holds one does not have to Daven Ma'ariv at night, as it represents the burning of the limbs and fats on the Mizbe'ach, which also did not have to be done at night and could be started during the day (see another reason ibid.). Similarly, Rebbi Yehudah holds that being that there is a Mitzvah of Tosefes Shabbos, one can start Shabbos from Erev Shabbos. He does not necessarily say that one can fulfill these Mitzvos at Plag.
[See Chidushim U'Biurim at length regarding which Mitzvos Rebbi Yehudah would say can be done after Plag and which cannot. However, there admittedly are different opinions as to how to understand Rebbi Yehudah. Whether or not Rebbi Yehudah holds this is the "beginning of the night" or not (like the Chidushim U'Biurim) seems to be contended, as is evident by the argument regarding Sefiras ha'Omer (see Mishnah Berurah 489:16 and Bi'ur Halachah there).]
4. He has transgressed an Asei of Tosefes Shabbos learned from Tosefes Yom Kippur (see Mishnah Berurah 261:19).
Kol Tuv,
Yaakov Montrose