Does karet mean the soul will not ever enter the olam haba? Can teshuvah rectify the transgression which lead to the karet? Can that soul in another re-incarnation rectify the blemish of karet? By "karet" I'm referring to what I've come across in Gen 17:14 and Exod 12:15.
To answer the first part of the question, the Rambam in the introduction to Chelek writes:
The punishment of Kares means they remain a physical entity, and are therefore unable to experience Olam Haba, and therefore they cease to exist along with the rest of physical existence.
Nefesh HaChaim (Shaar Aleph Perek 18) in his discussion of Teshuva explains what Kares is as follows:
And he goes on to explain that the highest level of the "nefesh" is not cut off from the level of "ruach," to which it is intertwined, and through that connection it is possible to reinstate the connection of the "nefesh":
He goes on to explain that Teshuva is what reestablishes the connection. So Teshuva repairs Kares.
In general, there is no sin for which teshuvah is not effective. See, in particular, Maimonides' Mishnah Torah The Laws of Repentance Chapter 1 Law 11: "עבר על כרתות ומיתות בית דין ועשה תשובה - תשובה ויום הכיפורים תולין וייסורין הבאין עליו גומרין לו הכפרה. ולעולם אין מתכפר לו כפרה גמורה עד שיבואו עליו ייסורין. ובאלו נאמר: ופקדתי בשבט פשעם ובנגעים עוונם - "If one violated a crime punishable by Karet or capital crimes and repented - the repentance and Yom Kippur combined with suffering that he experiences complete his atonement..." (paraphrased).