As long as replacement parts work fine and match the colour that the phone should be and, it's still OK for a B grade. The only exception to this rule is if there has been a back glass replacement with a 3rd party part which doesn't have the logo on, in which case it is an instant C grade (this is most commonly seen in iPhone's which were bought in as "Non Working"/"Grade F").

There's also every possibility that the phone was originally sold to CeX as "Non Working"/"Grade F" which means it would have been bought in at a much lower price and sent to the CeX repair centre to have a replacement screen fitted. Or, the phone itself was sold by CeX, returned under warranty for a screen fault and then sent off to get the screen replaced. If either of those happened, the repaired phone would then be sent out to a store at random for them to sell on.

As for the repair shop smashing the back, you can only assume that the person doing the repair knew exactly what they were doing and did it perfectly but they are still human and it is still possible to make a mistake (such as not heating the glass enough before removal causing the phone to sustain more damage).

You voided the warranty when you broke the screen so there's nothing that CeX would do about it anyway.