I think as an additional entrance that's already there, as the main Northumberland Street and the new St Georges Mall entrances onto the green and bus station would still be other main entrance / exit.My only concern for your plan would be in making a route round the back of Old Eldon Square vestiges. I don't think it could ever be made an encouraging route as it faces back of house on both sides.
Also, I just don't see the demolition of the northern section happening in our lifetimes, also don't John Lewis own their store, not Eldon Square / NCC. So that would need a hefty taxpayer compulsory purchase. Mike Ashley paid £31.6 million for the H&M / WHSmiths unit which is 90,000 sq ft; John Lewis is 298,605 sq ft; so that's nearly 3.5 times the size.