1. As has already been explained, one who remembers after having finished praying that he has not said “Vesein Tal Umatar Livrachah”, must return to the beginning of the Amidah, and when so doing, he does not need to repeat “Hashem, sefasai tiftach” as he is already in the middle of praying. (Ben Ish Chai Sh”Sh Vayikra 12)
2. If he realizes that he has to repeat the Amidah before the cantor has started his repetition of the Amidah, the basic letter of the law will tell us that he should listen carefully to this repetition and use that to fulfil hiys own obligation. When doing so he may answer Kedushah and “Amen” but not “Baruch Hu Uvaruch Shemo” nor “Modim Derabanan” nor the “Amen” on the priestly blessing. Some say that it is difficult to rely on this as generally speaking it will be very difficult for him to listen to the entire Amidah, and according to them it is better if he prays individually. (In this case, ideally he should pray together with the cantor so that he will not miss Kedushah.) (Shulchan Aruch and Mishne Berurah 124)
