Nazir 49a - Chazaka of being father

The gemara says that had the pasuk said by a kohen gadol that he cannot be metamei for his father, one can argue that it is because he is not definitely his father and purely based on chazaka, but a mother who is definite a kohen gadol can be metamei for (therefore it must also write "imo"). The Netziv asks, the logic is flawed. If the person that he believes is his father based on chazaka is in actuality not his father, then he is not a kohen. Therefore, if the assumption was that for a person who is definitely a parent, he can be metamei for, then he should be able to be metamei for his father 'mema nafshach' - if he is his father, he can be metamei [just like a mother], and if he is not his father then the son wouldn't be a kohen and can certainly be metamei for him?

The Netziv says a tremendous chiddush. Regarding the status of kehuna the chazaka is d'oraysa and definitely confirms the father to be the biological father, hence rendering the son a definite kohen. The logic of the gemara is based on an assumption that the pain of the loss is a contributing factor in allowing a kohen to be metamei for a relative. Therefore, one can argue that the pain is stronger for a mother who is definitely the biological mother rather than the father who is only based on chazaka. So, if the torah would only write that a kg cannot be metamei for his father, one would say that for a mother where there is more pain for the loss, he can still be metamei.

Obviously the Netziv is very difficult to accept that one feels more of a loss for a mother BECAUSE he is sure that she is his biological mother, as opposed to the father where there is some uncertainty despite the chazaka.

Perhaps the answer is based on the Ohr Hachaim (Emor 21:2). One can technically design a scenario where the son has a chezkas kehuna b'vadai, but is still only able to determine who his father is based on the chazaka. Meaning, his chezkas kehuna is stronger than the chazaka that the deceased is his father. Yakov and Reuven each have an independent chazaka of being a kohein [not based on their relationship to each other], and there is a chazaka that Yakov is the father of Reuven. Therefore, even if Yakov would not be the father of Reuven, Reuven would still be a kohein [perhaps based on witnesses who told him that they knew his father was a kohein, but did not identify Yakov as his father]. In this scenario it is plausible to suggest that if Reuven becomes a kohen gadol he would be able to be metamei for his mother but not for his father since the knowledge that Yakov is his father is based purely on chazaka. For this we need the pasuk of "imo" to teach that a kg cannot even be metamei for his mother.