Pretty good analysis. Its what I said before, couldn't NOKIA just say they are partnering to bring its innovations into WP without disrupting its current assets.

They could've had low-mid range some quality Symbian products.
Then some mid-range Lumia offerings.
Then perhaps a few high-end MeeGo offerings (would've at least kept developers).

If they realized WP wasn't selling as best as they hoped and that MeeGo was selling above their expectations, they could always just axe it or limit it to a few choices.

Later, they would completely eliminate Symbian as a whole ecosystem.
In place they would have MeeGo, which could be optimized (many software features removed) for low-end devices, and optimized differently for high-end devices. At least that's what Google's Android is doing, and its working!

When Windows Phone 8 debuts, Nokia can still be the "official" partner and sell them as mid and high-end devices. So it would be a 2-ecosystem plan for NOKIA just as they wanted, only that one ecosystem is closed and belongs to M$ and the other belongs to Nokia/Open Source.