Seems to me the issue here is NOT about whether or not a .243 will kill an elk. The issue is a hit in the vitals at 535 yards. Vital enough that the elk went only 50 yards and flopped over dead.
IMO, it means a spine or heart hit, plus the animal didn't panic. Those are danged small targets when 535 yards comes into the play. "Close" to either won't do a quick kill, IMO.
Again, my opinion: IF it was indeed 535 yards, and Daddy was there, Daddy ain't strong on hunting ethics about clean kills. Sure, luck is very hard to beat, but depending on luck isn't ethical.
You're from BATFE? Come right in! I use all your fine products!