Wildcat, there is no mechanism in the LOTG to ensure consistent application of the rules, in fact it promotes ref opinion and discretion in the application of the rules under the umbrella of spirit of the game. The Spirit takes precedence over whether the ref is right or wrong. Spirit of the game is only very subjectively defined as "The main/essential principles/ethos of football" although references are made to it including fairness and safety. If you asked 10 refs to define Spirit of the Game you would likely get 10 different answers (same with parents and coaches) but hopefully there would be some common themes. Spirit of the Game is a double edged sword, but I like the concept.
So Wildcat if you were a ref and you allowed multiple late minute subs because in your opinion that was fair then you would be 100% correct under the LOTG just like the refs that don't allow it under the perception of time wasting. Ironically as a parent you are wrong but as I ref you would be right.