Fuck yeah, I feel the same. Don't get it at all. xD
From my 2nd/3rd read, I interpret the events as Santa Klaus actually being a puppet created by Tolka's master. He did that same arm-transformation the other puppets did, and the mirror-panel of him and Tolka after the latter's been puppified by her further imply it. Apparently, under some conditions puppets can remain somewhat human. Her saying that he's now part of the family and calling Santa grandpa also support this idea. Or maybe she's a puppet made by Santa? Or anyone contracting the puppet devil becomes a puppet? Madness! Anyway, Santa then sacrificed himself and three of the children the German government gifted him (lmao) to summon hell into the mall.
.. but just as I'm writing this, I remember that it was said Devilmen can't be turned to puppets. Does this also mean a puppet cannot have contracts?
Also, that hell, I don't get it at all, but I know that I'm intrigued as fuck by the fact that the ceiling is made of doors looking like the one from Denji's dream, so that probably is a door to hell after all!
Also, does anyone else ship Aki and Angel..?