Yesterday, I finally got Keybuck's attention on my diagrams, and on the bug that they document (that if postrm fails, the package state is still "Installed"). After he finally believed me, both he and Ian Jackson (and I believe even Wichert Ackerman) started to review the code and discovered that this bug had been present in dpkg since it started, and they fixed it!
Now I'm known as the girl who broke dpkg by looking at it too hard :) And yes, I'll need to redo the diagrams, once the fixed package is uploaded, and hopefully I'll take the opportunity to add some other diagram, like a Conflicts or Replaces one.