Is there a date by which a transition to a new governance model should be completed?
There is. 
In the wake of the Jupyter Team Meeting in March, the intent was to utilize the remainder of 2019 to draft an initial governance model for review by the steering council.
Right now it is hard to tell, as someone interested but not involved, what the timelines are and by when a proposal is to be expected.
I would expect the proposal to come in mid to late December.
Other projects (like CPython) managed their transition in about 6 months (from Guido announcing his resignation to a new governance being in place and maybe even already elected).
The structure and community of Python are different in many ways compared to ours. We love their project and have learned a lot from them, but we operate independently and need to find solutions that are well adapted to our needs.
It is hard to explain to others (especially without a timeline to point to) why for Jupyter it is taking considerably longer.
If others are concerned, we encourage any party to attend the governance meetings to join our discussion and/or help on tasks. One issue the team has run into is that we built the structure of the governance committee around more participation than we’ve garnered at this time. The model for the process, as laid out in this discourse post, was to have Brian and Fernando act as an “Editors in Chief” model, to date, there’s been only a small trickle of content to edit.
The vacuum created by the lack of information is then filled with rumours and speculation (which is never positive) :-/
I haven’t personally heard rumors, but I would encourage anybody who has concerns or questions to attend the governance meetings (or use the discourse) and bring them up so we can discuss them. The process was intended to garner feedback, so let’s use it!