I drill a hole in soft or mild steel for whatever tap size is intended using the smallest size tap drill....in steel that will be hardened, a larger tap drill is used. I always used a ctsk on both ends of a hole before tapping......thought this was standard practice. On thin plates like a lockplate, the ctsk should be slight. The ctsk gives the tap an easier location to pick up and on the other side, eliminates a burr.
Lock bolts are made from mild steel and other than a bolt that's been hardened, shouldn't strip the threads.....the main reason threads are stripped is the thread depth is too shallow caused by an oversized or badly sharpened tap drill. Over tightening over time will also strip a thread as will a mislocated clearance hole.
I agree that if there's sufficient space, a plug could be installed and if not, the hole could be welded up......Fred