The rotor is just stuck on because the shoes in the parking brake are grabbing the rotor inside.
You will have to force it off; keep pulling and tugging on the rotor - side to side.
I understand that you have loosened the adjuster, but that does not mean the shoes have collapsed inside; sometimes the rust inside overcomes the springs and the shoes won't retract well enough. I also understand that you don't want to break anything in the brake assy; I get that. But it's unlikely that you'll do any real damage; you just have to keep trying.
You just gotta force it.