the safe zone extends horizontally from the switch as well as up and down, so providing the conduit terminates within the horizontal zone then no cover is required and your original idea is viable. ( depending on your interpretation you may need to earth the conduit if you don't consider it as satisfying the requirements for mechanical protection. )
ideally of course relocating the conduit so it is vertically above and directly feeding the backbox is preferable..
with regards to the coving and channeling behind it, a channeling bit for an sds drill is ideal, it's cranked and will go behind quite nicely..
alternatively you could hammer a screwdriver / rod etc through the plaster and up into the floor void ( behind the coving ) to locate it, then channel the last bit out from above if you can get access..
ideally of course relocating the conduit so it is vertically above and directly feeding the backbox is preferable..
with regards to the coving and channeling behind it, a channeling bit for an sds drill is ideal, it's cranked and will go behind quite nicely..
alternatively you could hammer a screwdriver / rod etc through the plaster and up into the floor void ( behind the coving ) to locate it, then channel the last bit out from above if you can get access..