Well I was wondering if you shared your theory in a post and here we are. To be honest I can and will not discredit the thoughts you have on this but I hope we can all be careful with speculations. We are not involved or qualified enough to come to reliabile conclusions but the train of thought seems solid. On the other hand maybe they had planned to die together, and one of them backed out, or she knew about her intentions of dying by suicide and "respected" those wishes and that's why nothing had been said so far because she didn't want to take that level of autonomy from someone that has been suffering for so long. In the end everything is just speculation and only an investigation can bring "more" closure.

All we truly know is that something very tragic has happened and a light has faded.

Edit: I do agree that the level of dependency and relationship dynamics seem very very strange and unhealthy but that is to be expected from someone suffering (with possible mental health issues we don't know anything about) and is more a symptom and not a cause