You are very brave. I'm trying to place myself in your shoes as I write this, and I feel empathy for you and what you are going through. You are strong to go ahead with the appointment, and I wish you all the best in this time in point.
In my own journey I've come to believe that timing is the most important factor for any action to take place, and also sometimes something seemingly unwelcomes happens for a reason. It may sound strange, but it's like the universe (timing) and or our inner person has to be aligned with whatever action we wish to happen. Sometimes the universe (time/chance) just interferes and takes things in another direction. And sometimes something just doesn't feel right (inner person). And so, we hold on as SI kicks in. I keep on postponing my date in my head, but I think (hope?) timing will align with my inner person, once I've made peace with my decision.
Hope I'm making sense. What I'm trying to say is please be kind to yourself, and we have very little, or no control over most things. How you woke up hearing your neighbours and your uncertainty about the note, may just mean that timing was out, and maybe for a reason we don't know of. But you also have more information now on the conditions around you at certain times during the night.
About the email/note, I'd consider just locking myself in the bathroom, e.g., with a note on the bathroom door to warn whoever and maybe an envelope addressed to the police taped to the door. After some time, some person is going to wonder where I am, come knocking and when there's no answer, I imagine someone will either knock the door down, or call the police to do it if the landlord doesn't have a key. I wouldn't use the main door because I may fail or it may raise suspicion.