Okay then, I included a TL;DR at the bottom since I spent too much time and effort on this and the post is 1.6k words long.
Alright, effort post (sort of) incoming. I'm in a good mood tonight because the Steelers embarrassed themselves in front of the country by losing to the Browns (congrats to anyone from Cleveland by the way) last night on national television.
Anyways I'm going to give my theory on what has happened here, why I don't feel a particularly strong urge to risk Mathilde helping these people, and try to give some commentary on the ideas I have seen from the thread. So we arrive at Karag Dum to find it surrounded by old forest that should not be there, it's neighboring mountains gone, and a growing desert in front of it, littered with bones. Out of that forest steps, Morghur, who as I understand it is to the Wood Elves what Hitler is to us, and not exactly well-liked by any other people. Borek who has waited for 185 years to come home sees this and is resigned. He knew this was a possibility, and does not try to bother to defend it to anyone, he only asked us to say that Dum tried their hardest and apologized to the Ancestor Gods. Then he walked up to Morghur across the desert, which I think is evil by the way, and gets a nice head pat from Morghur before he walks into the forest presumably to his kin.
Now I'm going to talk about a theory people came up with to explain this. Those were not beastmen, rather illusions of beastmen. This would be a pretty radical departure from normal Dwarven tactics, but Dum is expressly called as being radical. However I kind of doubt that Borek would ask the Ancestor Gods for forgiveness over this. Like I know SV has a weirdly paternalistic view that the dwarfs have too many emotional hangups, and can't be trusted to have an unbiased view of stuff, because they feel too much shame at everything, but given that Borek accepted what he saw calmly I do not think that is the case. Anyways next update we found beastmen bones which put that theory to rest. The beastmen were real and were friendly with Borek.
At the same time that that first theory was proposed there was a parallel theory, that the Dwarfs of Dum created a rune that could purify chaos corruption and Dhar on a massive scale. Personally, I thought that theory was wildly over-optimistic the moment I remembered that the Old Ones, and Slann exist and that Dwarfs are currently not as advanced as they used to be in the field of runecraft. Why would they suddenly develop very a useful capability now, that more advanced groups would have tried to develop. I can get the argument that even though current dwarfs are not as technically advanced as the dwarfs of old they could develop some novel abilities to suit new circumstances, except the rune of purifying Chaos would be the desire of three much more advanced civilizations who would gladly slap it on everything (Old Ones, Slann and Golden Age Dwarfs). I can't really accept that these guys would be the ones to create the holy grail of anti-Chaos magic when people who should have found it first did not. Moreover theories like this I think set unrealistic expectations, Dum goes from being a distant Dwarf Hold to being a distant Dwarf Hold with the salvation of Malus.
Anyways in the bone update we found out that the desert was slowly expanding, and that there was divine energy doing it that was not of the big Four. Now personally I don't know who is causing that, but considering that the only thing that the desert does not swallow up like quicksand is bleached bones, I'm willing to wager that they are not someone we want to meet. That screams bad news to me and on its own probably shows that there's something seriously gone wrong here. At this point we don't even know if Dum caused this, but considering that people who support their innocence seems to assume it was in the arguments I read, I'm just going to say that if thats the case, that makes these people less trustworthy, if Karag Dum made a pact with for example Khaine, who seems like the sort of guy who would like a skeleton desert, I don't think the excuse "it's not Chaos" is going to play well to the rest of the Karaz Ankor.
Now after the most recent update we found out that Morghur or a very convincing facimile of Morghur is capable of using Dhar still. No one has told us that the beastmen that let Borek through to Karag Dum peacefully, and Karag Dum have an alliance, but it looks bad. If Borek thought they were under siege and not safe with the swarm of beastmen outside, and that the convoy could be convinced that Dum was worth saving, he probably would have tried to get the convoy to help them. He didn't so either the Dwarfs are safe, or this guy who is definitely the sort of person who thinks that the "ends justify the means" would not even make an attempt to justify what is going on to a convoy full of outcast Dwarfs who could probably be relied on to give him an honest listen, since if they had a lot of animosity towards Dum in the first place they would not have joined in the expedition to aid it. That he did not attempt to explain himself tells me that he thinks that we cannot justify the actions of Dum. And if he thinks that this stuff can't be justified, then I think he's probably right. Dum either has enslaved Morghur, created a look-a-like or entered into an agreement with him. None of these are going to go down well with the Karaz Ankor. And before people start pointing to stuff like apparition binding or the mirror catchbox as an example of hypocrisy, as far as we understand Ghennas Golden Hounds were given to the Colleges by Teclis, and apparitions are explicitly non-Chaotic. Morghur is explicitly chaotic, and if he can still use Dhar he has not been purified.
Finally on the why I think Dum is beyond saving is that they have not attempted to communicate. Leaving yourself blind to the world is a bad idea for anybody, but it's really bad idea when your in the Chaos Wastes and an army of daemons can come knocking at your door. They don't need to come out and talk to us if they need to verify that that is really Borek, they can put up a signal flag asking us to hold our position while they verify that that is indeed Borek. They did not do this so I think we can assume they do not have an interest in communicating with us. This is really where I think I stopped caring about these people, if they don't want to talk to us then they certainly will not want to re-establish contact with the Karaz Ankor. The Karaz Ankor is not coming out here again regardless of what we find for decades at least. Like trying to establish an alliance with Karag Dum would be like making an alliance with people on the moon, only we can't use radios or other wireless communication methods. Everytime we would want to interact with them an exceedingly risky mission to send someone to talk to them would have to carried out. By the way in this scenario the guy who lead the project to build the first rocket died during the first launch, and no one else can really recreate or improve on it, at least until someone goes through the effort of learning how to do it again.
Finally I want to have some parting thoughts:
The idea that we could attach messages to rocks or arrows, and throw or launch them at the beastmen is built on a very shaky assumption. The idea that the immediate reaction of someone coming up to you and throwing a rock at you, while you are explicitly keyed up for a fight like those beastmen were is not to assume that it's the opening of hostilities. I think that if we had done that we would have had a dead ranger on our hands.
I think Morghur reacted positively to Borek because Karag Dum had Morghur before Borek left. I already talked about where I think they picked him up from, but I will speculate that the Dwarfs of Dum treated him like a dog, you know they taught him tricks, and as a member of the Royal Clan of Dum Borek would have interacted with him often since what's the point of "taming" the evil monster if you don't show it off. Anyways I remember Boney saying that the Dwarfs of Dum did not appear to armor him up, likely because they don't care for him because he's Chaos. Oh Shit I need to explain this (can you tell that this is very poorly organized)
So Karag Dums purpose is to stand as a bastion against Chaos and I think they are still committed to that. The reason they are sending energy through the Waystones is that if they don't their little desertification trick will be countered by the growth of the Chaos Wastes. Ultimately I think they are opposed to Chaos, but they have resorted to using means that no one will accept, and also do not want to talk to us. I think think we should leave, and when there defensive scheme inevitably breaks down it will be a moot point anyways.
Basically TL;DR Dum is lost, they haven't communicated with us because they know it, and trying to clear their name is a waste of time.