Good luck with the exams! First things first, do what you have to and then when you have the time come back. I've said it many times before but real-life is obviously way more important than the forum life and therefore the project!
since i cannot double post...i'll just tell you here. memories help form the heart. consumed and infused are two very different things.
memories help form the heart (confirmed in 358/2 days ultimania) and darkness help forms the heartless because darkness act's as it's body.
but unbirths (in my theory) are both darkness and heart fused together as if they were one (not seperated). heartless are just heart's covered in darkness. but unbirths could possibly be hearts fused with darkness.