If saying a superfluous Amidah results in brachos l'vatala, as suggested here ("However if one is davening after 1/2 hour after Chatzos he isn’t Yotzei and his Brachos are L’vatalah. See Biur Halacha Siman 89 Dibur Hamaschil V’Achar"), then what of a situation in which one must repeat all or part of the Amidah because of an omission? For what reason are those repeated brachas not l'vatala? (Must it be "because the rabbis said they weren't?")
Also, what about a tefilla nedava? Why are the brachas in that tefila not considered l'vatala?