As Cricko points out there is the covenant. Exactly what is acceptable as rehoming the club is open for debate.
Let me ask you this, with a buyer / backer as huge as Sainsbury's why do you think no progress has been made. My contacts at Sainsbury say it was always meant to be a joint venture with Sainsbury's having a very active part in building the new stadium, now whether that means financially or otherwise they will not expand on but it seems very strange that things have stalled again after they advanced £5M, my money is on not wanting to continue doing business with RM but wtf do I know.
You need a new source at Sainsbury's!
They scaled backed their expansion plans nationally a couple of years ago.
Also how water tight is the covenant ? ie. If RM was to say ok if we don't ground share and you let me build on Roots Hall the club will cease to exist, your choice, amend / scrap the covenant or the club goes under, if it came to that is it possible ? Please don't say he could have done that already, just because he hasn't doesn't mean it isn't an option.
The issue probably isn't about how watertight the covenant is but who would enforce it.
I think he was rather shocked that a rookie manager took us to the Championship so quickly and we hardly pushed the boat out with signings hence we didn't last long.
We may not have pushed the boat out by Championship standards, but we certainly did by the standards of a club of our size - Peter Clarke, Alex Revell, Billy Paynter, Richie Foran were all signed for sizable (by our standards) transfer fees and the likes of Hammell, Francis, Ricketts, Barrett, McCormack were given fat contracts (by our standards).
When we were then relegated we still had these liabilities (their contracts, rather than necessarily their playing abilities) on our books and we chased a quick return instead of scaling back to a sustainable level.