What is pshat in the gemoros question of rav mori: because he can do tashlumin for shacharis by mincha, this is proof that one could daven shacharis also after chatzos?
Farkehrt, since he needs to do twice mincha is the biggest proof that he cannot daven shacharis after chatzos...??!
Ronnie, belgium
I found your question (with which I had difficulty as well) in the Tzelach who asks that Rebbi Yochanan (on whom the Gemara's Kashya is based) confines his ruling to "someone who made a mistake and on someone who is Davening Minchah."
The Tzelach answers that the Makshan (the questioner) thought that Rebbi Yochanan was coming to teach us that if even under those circumstances -- where he erred until after he had already Davenend Minchah -- he is permitted to still Daven Shacharis (even though we might have thought that once he has Davened Minchah it is too late to Daven Shacharis), then how much more so if he did not already Daven Minchah. The Gemara answers that Rebbi Yochanan is referring not to fulfilling one's initial obligation, but that of Tashlumin (making up for the missing Mitzvah).
B'Virchas Kol Tuv,
Eliezer Chrysler