Starting the quest was fine until I got to the Falkreath entrance. I don’t want to post in the bug tracker yet since I just want to confirm these issues and go from thereġ. The only mod affecting it is USLEEP so I can't imagine it being a conflict aside from The Cutting Room Floor possibly doing something to the delayed burial quest.
I had not realized what a bug filled mess this questline was! I'll go into detail as much as I can.
I've never done the Dark Brotherhood quest before until now on any character, it never really fit with them but I decided this time to see it on a new play through.