There's nothing to compare with strange stuff like this!!
It is so much fun to just find things out.
But it usually it is a worthless diversion from matters one is pursuing, and so, a 'waste of time'.
Walnuts are known to exude stuff from their roots that prevents other things from growing near to them. Seems pretty wild to think that one variety can have an effect like this on another of the same species. But, as soon as you put these other seedlings in the same pot, your big tree went bug-fuckey, per your account. By this point in time, the seedlings you haven't removed are evidently fused and can get or are getting what they need from your big sick tree.
IOW, removing the tops may not have an effect if this is actually happening (i.e., root exudation). Nevertheless you can REMOVE THE TOPS OF ALL THOSE SEEDLINGS NOW and observe whether your big tree starts being more normal. Then, NEXT SPRING REMOVE ALL THE ROOTS OF ALL THE SEEDLINGS and observe whether your big tree starts being more normal.
Meanwhile, lets suppose that the big tree remains bug-fuckey despite all of this. Are you certain that you haven't censored any details from the story just because you 'know' it's not relevant? Think hard about it. It is nothing personal. It is something we all unconsciously do from time to time. If not, I have no further hypotheses and no further suggestions.
Well, I'll partially retract that. It is possible that your big tree will never recover - that it will be this way forever more. I am aware that some treatments can throw a switch in the DNA that cannot be undone. This is what witches brooms are. Your big tree's ability to synthesize auxin/gibberellin may be permanently gone. But, lets see if that is indicated.