If someone says the individual's Shemona Esrei and they can't remember whether they said the conclusion (which the chazzan's repetition does not say beginning Elokai Netzor) are they still yotzei saying the individual's shemona esrei?
Yes. That is just a customary addition (see Berakhot 17a that records this as the practice of an individual rabbi). It is found in the Siddur of Rav Amram Gaon (ed. Harpenes Seder Tefilla). This work underwent some evolution, but its core is from the ninth century. It is also found in the Siddur of Rav Sa'adya Gaon from the early 10th century, (ed. Assaf pg. 19). Rashi (11th cent.) (Berakhot 29b s.v. Tahanunim) similarly indicates that it is a customary part of the liturgy. Similarly, see Mahzor Vitry (89).
It is not listed as one of the 18 blessings established in Megillah 17a, or the additional blessing added by Sh'muel HaKattan recorded in Berakhot 28b, which form the required prayer text (see Rambam's Hilkhot Tefilla 2:2).