In fairness I think that everyone is aware of SISU's history, and Jon Sharp's no mug - CRFC are heading in the right direction. I don't think that there's any risk of them losing control of the rugby club through this.
Assuming there's any reality to the project, then the key for all parties is some kind of security of tenure if one side goes bust or decides to sell up and move on. I can't see why that can't be nailed down contractually.
As for planning permission, that's a big hurdle but I think the truth is that eventually developers almost always seem to be able to get their way. There is already a stadium there for up to 4,000, so the issue would be scale. I'm not commenting on the rights and wrongs of it, but look at Heathrow if you want to see how things can change!
Whether it's a good idea is a different argument. Personally if it were to happen I'd be happy with 15,000 at the Butts (or somewhere else that we owned or had a proper share of), and the place sold out every other week. I don't see that it's the size of the stadium limits our ambition, it's the revenue we derive from our place in the leagues and the willingness of the owners to invest that does that. Others differ, I know.