Again, just because something is extremely likely to be the case doesn't mean it is the case. First off just because the PM can alter humans doesn't necessarily mean it has and just because it alters other animals isn't enough proof that it's definitely doing the same since by that logic anytime something bad happens and people blame it on chaos than it must be true because Chaos can do it even though there are other factors involved.
Second, 40K human adaptation is weird as hell considering that a lot of variances show up including things like Beastmen and Ogryns. A major factor in Avernite human changes has been chalked up to the heavily warp touched nature on the planet. The fact that we are the fastest on the records doesn't automically mean that something is involved directly rather than other factors.
Third, to the above examples there is a collosal difference between having very strong suspicions that some thing is true but not having any absolutely concrete evidence and saying that we absolutely know something is true without actually having absolutely concrete evidence. For example we can say 'We have very strong reason to believe this but can't say for absolute certainty due to circumstances and lacking absolute concrete evidence'.
Saying that we absolutely sure that an alien planet mind is altering humans is likely goint to cause issues. Saying that we have strong reason to believe that it's happening but lack absolutely solid proof would make it easier for everyone else to look the other until there is absolutely solid evidence directly linking the PM to the alterations considering how valuable Avernus is and that fact that trying to pick a fight with it is suicide.
To the first I ask why the hell would it not? It has no qualms about altering other sapient species, and (the other Avernite peoples confirm it does) and no that isn't good logic. Bad things happen and can not be chaos because there are
other reasonable explanations. There is no other reasonable explanation for this, it is such a statistical and improbably outlier that it convinced the most paranoid group in the Trust. In there is a lot of evidence to say that it would, could and is currently doing it and not 1 IOTA to suggest it isn't.
And no
both the examples you give are wrong. The Ogryn are off shoots of the Jotun which surprise surprise were created artificially in Helheim. The Beastmen too are generally accepted to have been created by the DAoT for reasons unknown. And yes it is weird, because if it were
warp warp mutations doing this then we'd be shoggoths right now! This isn't a 1 in a million chance this is a one in infinity chance of this happening due to warp exposure and daemon not being involved, and if it a daemon then I guess we'd better do a mass suicide.
And no being the fastest on record, combined with the stability of the upgrades, the lack of detriments
and the fact that nothing in the universe that we know of can make it more likely for a species to produce more psykers genetically is my reasoning from it (warp exposure only increases the chance of a psyker activating their powers it does not make them more likely to have them). But guess what there's something that can do all of those things and we live on. On their own without the knowledge that we are standing in the place where orks were prototyped yes it would be a massive leap of logic HOWEVER, we are doing just that.
There's also the concept of Occam's ****ing razor, which states that if you have two explanations for something take the simpler one. Your examples so far have been "random chance in 100 trillion" and "impossible random chance" mean while mine works off the reasonable and simple supposition that we're on an Old One laboratory planet and we can't exactly refuse.
And saying "sorry we dunno" also causes issues, because the unknown is significantly more scary and that leaves open your idea of it being caused by the warp=Daemons=chaos= AHHHHHHHHHHHHHH! At least the high council knows Avernus hates chaos with a passion it's at least a semi known entity with fewer negative connotations.
At this point I'm reasonably confident you just don't want to concede the argument.
"There isn't concrete evidence" is not a good explanation to not accept something, you're pulling on the same logical positivist, verificationist logic that demands I disbelieve all historical events, the moon landings and anything somebody else tells me that I can't verify absolutely 100% verify, in other words not great logic. It also fails because if someone says "there isn't evidence to prove this 100%" I ask two questions.
1. Why do we need 100% evidence.
2. Where's you're proof against.
There's plenty of proof for, I've been giving it too you, but you've offered no proof either besides theorising of which in my opinion are far more unlikely than what I propose with even less evidence.
This is not helped by the fact that other people are going to make the "leap" in logic that I have based on the evidence outlined on their own (one very important group already has) and don't you think that taking into account Occam's razor the massive massive evidence all pointing at it and saying "THIS THIS THISSSS" and the desire to get a better explanation than chaos because they're really the only other reasonable option (Well it COULD be the necrons. Or Eldar. Or krork, but as there's no concrete evidence to prove or disprove this I guess they're all equally valid as well huh?) I'd say being final on this is fair?