What you are saying is WRONG. Even on the most chomer aveiros including murder, eishes ish, etc. one can do complete teshuva.
Many people somehow have a wrong hashkafa on this matter.
Incorrect. Once you brought a Mamzer into the world thres no undoing that. Although there may be some way to do Teshuvah Im not sure theres a 100% Teshuvah for making a Mamzer.