Ah, don’t get me started on the bus station! I don't know the whys and wherefores of why M&S don’t have an entrance there but I feel like it should have been contractually obliged to do so in taking over the unit.
Re the railway station, it ideally would be something vaguely Meadowhall like to better serve the shopping centre
I suspect that the bus station access was closed for security reasons. Presumably there was nothing in the contract preventing this, or it was renegotiated when M&S took the unit on.
From M&S's perspective, having that entrance in place means additional security staff and equipment, and impacts store layout. It would mean increased footfall (as bus users would walk through to access the mall) but any additional revenue generated won't necessarily cover the additional costs including greater losses through theft. The M&S unit has an additional exit to the car park upstairs, and the staff all seem hyper-vigilant* to it being used by would-be shoplifters. The bus station exit would be an absolute nightmare for staff.
Of course, closing the entrance just increases the other problems with the bus station, but that isn't the retailer's responsibility.
Something akin to Meadowhall Interchange would be ideal, but I'm not sure how we could get anywhere near there from where we are now.
*= The other week I visited the mall and parked close to this entrance, using it to access the mall via M&S as I needed to visit Timpsons at that end of the shopping centre. However I neglected to pick up the keys I was having duplicated from my glove compartment, which meant I was seen by the staff entering, then leaving, then re-entering in the space of just a few minutes. The swiftness with which the staff engaged with me suggests that they're used to organised gangs targeting that exit as a "weak spot".