
I think that the true older brother is just MC again, but from a third timeline, where the first one screwed up the second timeline by not convincing the child MC to follow him, causing ark to gain more power through completing their research. Also they probably screwed around with the dungeons and potentially fostered the world-ending dungeon creatures? So then the gate in the first timeline was meant to be D-rank, but was cultivated by ark which powered up the creatures inside? No idea why they'd do that, or why they'd need child MC for that. Anyway, I feel like it makes more sense to use the characters we've already been shown rather than insert a new brother character. Also sus that they didn't show his face and he made excuses.
Also random, but I feel current MC's identity is going to be revealed due to having the same fingerprint as the child MC. That one chapter where they got MC to sign with his fingerprint felt like it's gonna be relevant again at some point

Did they explain why the skeletons were feeling itchy, rash-like symptoms in the pax room?
If they didn't then I'm making it canon that the auto-healing ability is regenerating their flesh and we'll get 12 handsome men and or women. Amen

Since they're undead, it's more of a slowly harming them kind of thing. Treat it in similar to feeling heat from a flame when you get too close, it feels hot and the closer you get, the more uncomfortable until it begins to hurt you. In that same sense is how the auto-healing which is a trait under "holy" that would be destructive to undead elements. The knights are specters, which is a type of spirit that isn't alive. Healing is meant for the living. Undead may be dead brought to life, but it's a "cursed" life, so the healing for the living is registered as holy damage to impurity.
Even if we'd like the idea of them becoming human, sadly this isn't possible without an ability to create, rebirth/life, or able to change something. Maybe if someone pops up with an ability of this level, that would be cool.
Nah because why do I kinda feel bad for this old man. For the love of god someone please talk to him and let him do his coy act