Could be wrong, but I thought that the twin only policy at Sheffield was a local authority planning imposition. Councils knowing as much as they do about GA, decided that there was insufficient clearance for a single engine aircraft to glide to safety in the event of an EFATO. I suppose this comes from their years of experience of flying in twins on one engine
Yes, Sheffield could be a great GA and exec airport, but I suspect that Peels want it as a sweetener to smooth through the planning approval for Finningly, and will dispose of it one way or another, after the appropriate period of time is up.
I cannot see GA being too much of a problem at Finningly, there's loads of space for everyone, and although there are big plans for the place, I can't see them reaching capacity with commercial movements to the detriment of GA for a number of years.