Kagaya swaps with Muzan in this AU, but him and Amane are still married- in fact she's the second demon in existence, and they were married before he became a demon. It was actually her encouragement that caused him to allow more doctors to try and survive.

He can also still see into the future like canon- he can also just do the usual demon things and create more demons as well. Amane's BDA actually replaces Nakime's in a way. It allows her to create a maze of seemingly endless connected estates, complete with basements, gardens open to a sky of darkness, bedrooms, everything one could think of for their ever growing family.

Because that is what the demons are to them. Yes they're also a means to an end, but the two of them are Very Much platonic yandere if that makes sense. They weren't able to have children in their human lives no matter how much they tried, and can't as demons (Part of the want for the blue spider lily is to study it to see if they could change that, alongside the sun immunity). So they're creating a family. Their family.

Urokodaki & Kuwajima are actually from the Kamakura era in this AU. They were rivals of sorts as humans, & known as the Tide & Rumble demon in the corps' records respectively.

Urokodaki's BDA actually allows him to turn into liquid- usually water but this sometimes will also include blood. Meaning decapitating him is actually Useless. He still lives on a mountain sometimes, and actually raises several children still. Just in this case, they're usually half-demon children, who can't stay in the Hearth for too long because of human needs. Sometimes he's called down to gather information or make someone disappear as well, though this is less often as more demons are created.

Kuwajima, I'm still figuring out BDA besides the name being Thunder (how original, I Know) but he actually disguises himself as human every few years or decades for long periods. His human forms are usually known to the slayer corps actually, usually as a retired swordsman or similar, which the demons use to their advantage, slipping in one of his students every once in a while to learn where things have been moved and other information. Of course that means these students must be human, but I'm sure there's nothing to worry about there.