These type discussions happen when people try and split hairs, that are imaginary and likely NEVER TO HAPPEN. Only an idiot would allow a play to get off, with only one team on the field, no matter what the reason the opponent remains on the sideline.
When someone is intent on being STUPID, you have to deal with THAT problem, which is likely unique to THAT situation and THAT game. Hopefully a stern admonition will suffice, but when that fails additional action may be necessary. For those insisting on a specific WRITTEN rule to justify every action, in the case of ; When having alerted each team a charged TO is about to end, and either/both team(s) delays in returning to the field promptly, declaring the ball RFP is a logical, practical incentive and starting the 25 second count.
If the defense fails to respond, to repeated persuasions, and the offense appears ready to initiate play, the Referee can (and should) interrupt the count and kill the ball to prevent an undefended play from happening. Depending on the circumstances being dealt with IN THAT GAME, The Referee has several options at his disposal, including, at his discretion:
1. Charge the offending team with DOG (NFHS 3-6-2-f) Action or inaction which prevents promptness in putting the ball in play. (f) Any other conduct which unduly prolongs the game."
2. Charge the Head Coach with USC for violating NFHS 9-8-1 ("Examples are, but not limited to) although b, c or d would likely suffice.
3. Charge the offending team for violating NFHS 3-6-3, although this would likely be a VERY last resort, considering the authorization stated in NFHS 1-1-10.
4. Decide on another action authorized by NFHS 1-1-6 "The referee has authority to rule promptly, and in the spirit of godd sportsmanship, on any situation not specifically covered by the rules. The referee's decisions are final in all matters pertaining to the game."
As for rule reference regarding the Umpire positioning himself over the ball, after (or before) the RFP, you might consider NFHS 1-1-4 "The game is administered by game officials whose title and duties are stated in the NFHS Game Officials Manual, which indicates "Player safety is the first responsibility", which would clearly be threatened by the potential chaos created by allowing a team to initiate play BEFORE the opponent was ready to defend.
Of course these types of speculative quandrys are mostly, and usually, eliminated by the reasonable level of mutual respect and cooperation extended, consistently, by each of the adult supervisory functions associated with the game, to each other.