Here's the entire 8-6-16b:
Any coach or member of the offensive team, other than a runner, interferes with a defensive player's opportunity to make a play. This includes, but is not limited to: b) A coach intentionally interferes with a thrown ball while in the coach's box, or interferes with a defensive team's opportunity to make a play on another runner.
My emphasis. The thrust of 8-6-16 does not require intent nor is it limited to subsections A-D.
You are correct that the on-deck batter may leave to direct runners advancing from third base to home. This brings the on-deck batter close to the play much like a base coach, except there is no coach's box.
Regarding the On-Deck Batter:
7-5-4 The on-deck batter shall not commit interference with the defensive team. Penalty 1. When the interference is with a thrown or pitched ball, the ball is dead and the runner closest to home is out. If no play is obvious, no player is out, but runners shall return to the last base touched at the time of interference. Penalty 2. When the interference is with a fair batted or foul fly ball, the batter is out. [...]
Intent is not required.
The OP stated: "With a runner at first base, the batter hits safely to center field. F8 overthrows F4. The overthrown ball rolls into foul territory near the on-deck circle and hits the on-deck batter. There is no obvious play on either runner."
The on-deck batter is normally in the on-deck circle doing warm-up stuff. The on-deck circle usually has near perfect vision of the field and the action. An on-deck batter paying any attention whatsoever would never get hit by a ball thrown from center field. It's simply incumbent upon the on-deck batter to stay out of playing action.
Yet, it happened to the OP's on-deck batter, a member of the offensive team. Suppose the ball deflects away in live ball territory and runners keep advancing. How many bases should they be allowed to take? All of them? None?