I'll be honest, I think the NRM think too highly of Mallard to want to steam her again. Whilst Flying Scotsman should hopefully be out and about in a short while, what would they have to stand for at the actual museum if their two most notable locomotives weren't even residing there? Think about it.
In my opinion, it's either one or the other. If Flying Scotsman is out, and Mallard is in, then they'll be safe. If Mallard is out, and Flying Scotsman is in, then they'll be safe. If Flying Scotsman and Mallard are both in, then they'll get just as many visitors, in theory. But if neither are there, there won't be so many people attracted to the place.
The NRM spend so much time using Mallard and Flying Scotsman for publicity, etc., if both weren't even there - the place could lose it's purpose! What else is there? - fair enough there's the Duchess of Hamilton, but she's nowhere near as popular with people as either 4472 or 4468 are! People aren't going to come from all around the world to not see the two most famous steam engines in the world, are they?
While I would absolutely love to see it happen, from a different point of view, it ain't gonna!