From what I understand:

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Shadows of evil - Richtofen gets summoning key
Giant - Richtofen kills his older self and puts soul in summoning key
Der Eisendrache - Dempsey kills his older self before he gets to the moon
Zetsubou No Shima - Takeo kills his older self

The reason the characters are killing their older selves is so that maps like Moon, Shangri La, Ascension and Call of the Dead, Mob of the Dead and so on don't end up happening in attempt to stop the zombies. There's loads of theory videos which explain it much better

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The ZNS EE is nowhere near complete, blundell said it cas more complicated than DE but it doesnt seem that way.
On Reddit ATM there are two teorys
1) there are other steps after the cutscene (even if we get the thropy tho)
2) the Aywhere but here goblegum required to get a piece of the elevator is a glitch, that isnt the way to get that piece
(for who doesnt know, to get 1 of the 3 pieces you need to keep teleporting using a specific goblegum until RGN teleports you in an out of map area in order to get that piece, after getting it you can just jump out of a window and get back in the normal map.
I am thinking the second teory is right, after all blundell wouldnt lie to us? 

I kind of disagree. I would put money on the MAIN Easter egg being complete, although I'm sure there's a Katana somewhere in the map as we are missing a melee weapon. Also I would imagine the Anywhere but here gobblegum is the correct way of getting into that room and the only way, after all it's an unlimited gobblegum.