I don't understand their question: Can we still say K. Shema, even after hatzot? What is the nafka mina if they do say it, i.e., there is no issur - the quest- tion is whether they are yotze yedi hova? But because it is a din d'orita, and safek d'orita l'hachmir - there shouldn't be any doubt at all whether they should recite K. Shema. However, that does not seem to be the question, rather, should we, or shouln't we, recite K. Shema. Can you explain? Yashar Koach.
David, Hebron, Israel
As Rashi (DH Kavasi) explains, Raban Gamliel's sons asked not if they were allowed to read Keri'as Shema after Chatzos, but if they were obligated to do so.
It is true that had Raban Gamliel not known the answer to his children's question, his children would have had to recite Shema mi'Safek because of the rule that Safek d'Oraisa l'Chumra. However, this principle only applies if one has not been able to clarify the Halachah. In the Gemara's case, Raban Gaamliel did know the Halachah, and therefore there was no need to apply the rule of Safek d'Oraisa l'Chumra.
Kol Tuv,
E. Chrysler