1. All Rav Heller says is that it counts as a prayer. So he gets a merit but what about the demerit of not praying when the Halacha tells him to?
2. As for preparation, what would a king say to someone who excused his chronic lateness for audiences by saying that he had to practice his speech? It was because of things like this that he Gra put them in cherem. BTW, in “All for the Boss” Ruchama Schein mentions that her father once asked a rebbe why he did not daven according to the zemanim. He replied that he was ill and was not able to daven on time. Rav Herman then asked what was the excuse of his chassidim. The rebbe replied “Chazal say that if someone has yirat Shemayim people listen to him. So you tell them”.
3. Even if someone started before chatzot it would have to be an awfully long Amida to take him almost to Mincha time, especially in the summer. Perhaps they are dyslexic.