Thanks for advice Jaybird
Here's an update. Called the dealer I bought from. He's out of state. Though not unpleasant, he was insistent nonetheless that my cable routing was wrong. So I checked the book, routing was correct. Called him back, he gave me direct number to KTM to see if it might be a known problem. Haven't called yet, but on re-examination found an ever-so-slight kink in the area along the frame backbone where it runs under the tank. I carefully massaged this out with the flats on a pair of needle nose pliers. This helped, but throttle still would not return completely if bars turned fully to the right. Against my better judgement, I decided to lube the cables. Disconnected them from the throttle pulley, and checked the mainspring at that time, which was clean and properly installed. Held the cables upright, and used a dropper to drip in some Mobil 1 5W-30. Allowed 10 minutes for the oil to drip down via gravity. Then operated the cable. Reinstalled to the throttle pulley, adjusted the cables, and it seems to be working fine again. These appear to be old style cables, so I don't think there is any harm in lubing them. I had no alternative regardless. A 520 with a sticking throttle is not good.