Is there an opinion that holds the time for Krias Shema is sunset?

Question

Based on my research, the opinions that I have found state that the time for reciting Shema is Tzeis HaKochavim.

I have found the opinion (namely, the Rabbeinu Tam) that one can potentially fulfill Krias Shema after plag but a) I’m not sure if he means only b’dieved and b) I am aware we don’t hold like that option.

My question is:

A) does anyone hold the time for Krias Shema is initially shkia (lchatchila)

B) does anyone hold that bdieved you were yotzei if you said it after sunset? (Besides for the Rabbeinu Tam’s opinion mentioned above)

Answer 1

The very first mishna (Berakhot 1:1) sounds indeed like nightfall is the time, but the gemara there quotes a few other Tannaim who had slightly different times, including שעה שקדש היום בערבי שבתות when Shabbat starts on Friday evening. There's a lot of discussion about when exactly all those times are, which is earlier or later, etc. Arguably if not plausibly, one or some of them are at or around sunset.

That opinion of Rabbenu Tam (shema after plag) is found in the very first Tosfot (Berakhot 2a). There, Ri is also quoted as allowing relying on those other Tannaim for the earliest time for reading shema. So, arguably if not plausibly, Ri fits the criteria in your question.

It should go without saying that many subsequent authorities were unhappy with relying on these leniencies.