Abbreviate Argumentum ad Hominem to AAH. For consistency, I use 'credibility' to mean both believability and credibility. Source: p 133, A Concise Introduction to Logic (12 Ed, 2014) by Patrick Hurley
Here is an example of an argument that discredits a witness:
[1.] Mickey has testified that he saw Freddy set fire to the building. But Mickey was recently convicted on ten counts of perjury, and he hates Freddy with a passion and would love to see him sent to jail. Therefore, you should not believe Mickey’s testimony.
[2.] This argument commits no fallacy. The conclusion is not that you should reject Mickey’s argument but rather that you should reject his testimony. Testimony is not argument, and the fact that the witness is a known liar and has a motive to lie now is relevant to whether we should believe him. Furthermore, note that the conclusion is not that Mickey’s statement is literally false but rather that we should not believe the statement. It is quite possible that Mickey really did see Freddy set fire to the building and that Mickey’s statement to that effect is true. But if our only reason for believing this statement is the mere fact that Mickey has made it, then given the circumstances, we are not justified in that belief. [3.] Personal factors are never relevant to truth and falsity as such, but they are relevant to believability.
[4.] Yet there is often a close connection between truth and believability, and this provides one of the reasons why ad hominem arguments are often effective. In evaluating any argument there are always two issues to be considered: the quality of the reasoning and the truth of the premises. As noted, both are irrelevant to the personal characteristics of the arguer.
[5.] But whether we
acceptthe premises as true may depend on the credibility of the arguer. Knowing that the arguer is biased or has a motive to lie may provide good grounds for distrusting the premises. [...]
I summarise - as 6, and  as 7 and 8::
Truth and falsity must be distinguished from credibility, as AAH does not affects the former but affects the latter.
AAH affects credibility.
- Credibility may affect our evaluation of the premises.
To me, 4 appears to mean: 9. But our evaluation of the premises affects our evaluation of the quality of reasoning and the Validity and Soundness of an argument.
So Modus Ponens applied to 7-9 and Hypothetical Syllogism produce:
10. AAH affects our evaluation of the quality of reasoning, Validity and Soundness.
Is 10 correct? To me, 5 contradicts everything above it.