The more my headcanon of Dark as an usurped and exiled Hellgod makes sense out of the mindfuck that is WKM.
The bad vibes Celine felt in the house, and everybody going crazy? That’s Dark, alright. More accurately, it’s His demonic what-passes-for-blood. His battle wounds were leaking His energy. The universe of WKM was immediately adjacent to His own, and was the first one He jumped into to save Himself from… well, not dying because He’s immortal and all… but there are worse things than death.
Imagine if Dark was like a vehicle. Basically, He cracked His engine block and was leaking petrol all over the place. Petrol that everybody breathed in the fumes and went bonkers, and it almost exploded everything. Fortunately, He’s (mostly) repaired now. Too bad nobody else survived it.
The house not obeying the laws of physics? A Hellgod’s very presence can warp reality, especially when He’s too weak at the moment to rein in the effect He has on everyone.
The possession of Celine? Dark’s first attempt at possessing the beings of that universe, but they were too fragile to contain Him. He couldn’t stay in hiding there, because His very existence was wreaking havoc. So He had to find another world… and He did. A similar one but with slightly sturdier people, where He could lay low for awhile, until He got His strength back.
Your/our hand warping into Damien’s? Dark can alter reality without really thinking about it, so it’s not like shape-shifting a body He’s taken possession of is a problem. He just needed to hold a shape long enough to jump into another dimension/timeline.
Celine and Damien’s not-quite death? Dark consumed their souls to repair His True Form and, thanks to that, took on some of their characteristics… for a time. What little remains of them is a part of Him now… and the recollections of their memories and behaviour helps to smooth the path in acting more convincingly human.
So, there ya go. The headcanon still works, and we don’t have to ignore WKM completely for it to make sense. Don’t say I never gave ya nothin’. 😛