Sorry for the late reply! And a big thank you for your interest in my game! Let’s see if I can help you out.
SPOILERS BELOW
You are 100% on the right track! The sliding door is a two-part puzzle: one part is the flesh which you already burned off and the other is that the portal symbols around the door are smudged and don’t work anymore. Because you read the diary, Theresa should know which symbols are supposed to go around the door, so it’s just a matter of rewriting the symbols to make sure the portal works.
Hope that helps! And if it doesn't, feel free to reply again!