Firstly an AP paddle will look 80% worn from new, so if the garage fitting the box were only used to seeing/fitting standard stuff they would not be any wiser and would think its worn. Dont worry.

The gear crunching in reverse is either a misalignment of the linkage, or a knackered box. If it struggles when your driving forward, again it could be linkage /box problems or possibly the engine or gearbox mountings.

Go back to them and ask them to re align it, as this is the cheapest option first, tell them your not happy and if the re alignment does not solve it then can they advise you on what the problem is.

They will possibly try to blame it on the clutch so be aware, but a worn clutch will not make the box struggle for gears as you suggest.