The reason why the spear hit Schreiber is because it was allowed to. Otherwise it would just bend right out of the way.
And, honestly, this part I am rather unsure about myself. In Kei's route, we see a "regular" bayonet harm Reinhard. But before all that, we see Eleonore not being capable of harming Reinhard's body. Only Machina can do so due to how his Ber'iah works.
And this is rather important to note: The narration explicitly said that Kei's sword lacked the POWER to do so. And then you have Valeria's comment about the Legion having some ridiculous attack power. But for the sake of consistency, it is most likely for the best to simply chalk it up to indeed be a weakness of Reinhard. Or maybe there is an argument to be made for either side.
KKK is a what-if route that happens after Marie. It IS canon though that Hajun VS Dies Irae happened though as far as I am aware. It is just that Snake made it impossible for Hajun to appear after whatever shenanigans ensued that let them survive by removing any and all possibilities that led to him existing.
And the funny thing about Reinhard's own power destroying him? It is also mentioned that this is due to him attacking Mercurius. The whole "cancer cannot live without its host" thing.
And the whole resurrection/Regenerationn thing? Yeah. I agree that Reinhard doesn't do it instantaneously. But from what I could tell? Solely because of the Swastika limiting him. Might be stretching a bit, but it did apply to his lance sooo YEAH.
And no, Wilhelm doesn't literally consume the concept of escape. What it is really saying? You are now surrounded by a fuckton of stakes. Where in the bloody hell are you supposed to go without being skewered?
Cue Schreiber roaring and solving the problem.
But I do find DI's speed ratings to be ridiculously inflated at the moment, so I wouldn't mind seeing yet another revision on that front.
Or we can toss our hands up and just say that DI is full of inconsistency. I'll have to rustle my memories and see what else I can bulls-- I mean, figure out.