I think I know Carol's deal now.

For one, it's abundantly clear that the family is deeply religious, and deltarune's predominant religion worships the Angel. The Angel is heavily implied to be us, and we are the light inside the SOUL.
So of course she's okay with whoring her daughter to us. Of course she collects thousands of heart pillows. Of course we are always allowed in her house. We're literally her God. She worships us. That's why her house is always covered in Christmas stuff, it doesn't celebrate Christ, it celebrates the Angel. That's why you find it on top of the tree.

That's why she loves us, and will do anything we want.