The gemara 32b concludes that everyone can be mochel on their kavod except for a king. Yet, the gemara on 33a says that R' Nachman would not stand up for gentile elders because "if not for torah, there would be many nachmans in the market". Rashi explains that the reason he did not stand up is - אין חשיבותי בשביל עצמי אלא בשביל תורה. This sounds like what the gemara thought originally that Torah does not belong to the Talmid chacham and he has no right to be mochel, but the gemara concludes that תורה דיליה היא. Based on this conclusion, R' Nachman could have been mochel on his kavod, so why didn't he? Perhaps the peshat is that the gemara says that although a chacham can be mochel, hidur still needs to be done. Meaning, some small symbolic respect must be shown. It seems clear that if the mechila would not help completely even for the hidur, certainly it will not help for things which are clearly degrading. For R' Nachman to stand for a gentile elder would be degrading for the kavod hatorah, this he could not be mochel on.
However, in regard to kibud av the gemara says 31b that R' Yakov allowed his mother to serve him. Seemingly this was also not just mechila but even degrading. Furthermore, Tosafos 31b quotes a story where R' Yishmoel was told to allow his mother to wash his feet, which was certainly degrading - how does mechila help for this? The answer is that Tosafos is meduyak that the rationale is not mechila, rather it is that her will is her kavod - רצונה זהו כבודה (תוס' ר"י הזקן). Mechila would not allow for bizayon, but if her desire and will is to do it, that is considered her kavod.
There would still be a question on this approach from the gemara 32a where the gemara says that for Raba to get angry at his father R' Huna for tearing the silk would not have been a violation of kibud av, since he was mochel on his kavod. Seemingly, getting angry would be degrading, not merely abstaining from kavod, yet mechila seems to have helped for that. Perhaps there is a difference between a talmid chacham and a father - for a talmid chacham mechila doesn't help for bizayon, but for a father it does. Any thoughts?