on 3a on 1st tosfos, according to Rashi on 2b 2nd from bottom, rashi says could mean person sits to eat during daytime, but if so why doesn't gemora give that answer on top of 3a, ie, that R' Meir 2 answers are same time! because then that won't conflict with R' Achai.
You write very concisely, and I am not sure if I understand your question properly.
It seems that you are asking how to answer the question of Tosfos 3a DH Kashya, according to the first explanation of Rashi 2b DH Nichnasin.
(Tosfos' question is, why doesn't the Gemara suggest that a person eats Erev Shabbos at the same time that a Kohen goes to be Tovel. Tosfos' answer is based on the second explanation of Rashi, that Rav Acha of the second Beraisa is referring to the time when people eat on Erev Shabbos , so his answer will not be valid according to Rashi's first explanation, that Rav Acha is referring to the time when people eat on a weekday .)
If this is your question, I would point out the following-
(a) Tosfos on 3a does offer another answer which will be valid even according to Rashi's first explanation.
(b) In addition, please note that Rashi 2b ibid. writes that the time a person eats on Shabbos night is later than Tzeis ha'Kochavim. (This culinary fact seems to be a given according to both explanations Rashi offers for Rav Acha's words.) However, the time a Kohen is Tovel to eat Terumah is before Tzeis ha'Kochavim. If so, according to Rashi there is no need to look for another answer to Tosfos' question. The time a person eats on Erev Shabbos cannot be the same as the time a Kohen is Tovel, since one is before Tzeis and the other is after Tzeis. (See also Tosfos Rosh, 3a.)
Best wishes,
Mordecai Kornfeld
Kollel Iyun Hadaf