"if you hear the doorbell, please open the door as I'm expecting a parcel."
this isn't "if you aren't indisposed" - it's "if you hear it"
"I actually assumed if she heard the bell she would open it anyway as I think anyone living under a shared roof would automatically do so." But I asked specifically as I am 1. Deaf so often miss the bell, and 2. Have a newborn so am often trapped under a boob monster. "
Right, so she is actively asking for more than just "if you're around"
"Anyway, she said that when she is not officially on duty, she will not open the door unless she happens to be walking past or making a cup of tea (etc). She said she won't leave her room to open it. "
So - if not indisposed she will; she won't rearrange her private activities for this.
"I couldn't believe we were having this conversation, but didn't want to lose the plot already being deeply hormonal and emotional."
yep this is a problem here (I've been there!)
" So I just said - "okay please let me know when you're having a period during which you can't open it so I can make sure I am near the bell," and she said no, she didn't know when the mood would hit her not to open it so she didn't feel able to pre warn me."
No, she doesn't want to let you know when she's going to her room in her free time. or basically negotiate for free time (in her free time)
What OnlyLovers is presenting as the logical reasonable thing to do is what the AP has offered to do, but the OP has asked more and isn't admitting to it by fudging