Your arguments are: 1) Trans girls have to endure mocking and humiliation so we should just let them keep their athletic advantage, and 2) We should overlook fairness because in the grand scheme of things, it doesn't matter much.
1) Irrelevant to the issue of fairness on the field.
2) Disagree. You could use the same argument to argue against age separation. Why shouldn't a U18 play U12 if they want to?
1. No, society's interests in terms of what is and isn't fair in life cannot be limited or ignored by drawing chalk lines around a patch of grass and proclaiming that only those things that you want to happen inside them can be used to determine what is and isn't fair. In reality, there is nothing that makes the "sanctity" of a children's game so important that it justifies ignoring every single societal interest that supports letting trans kid play. In reality, you are just using the term "fairness" because that sounds better than what you really mean, which is "ignoring every societal interest that conflicts with my hatred of trans children and my fear that my daughter may never win a trophy."
2. Uh, this is exactly why most state HS federations allow 17-year old HS juniors to play JV against 13-year old freshmen. It is why I've seen a league allow a new high school with mostly freshman and only a handful of sophomores field a varsity soccer team and regularly get their ass kicked by 18 year olds. It's why 13 year old freshmen regularly compete in varsity sports. It's the reason one of my kids was often "deprived" of medals in 5k growing up because he had to compete in U18 age groups. Leagues allow things like this to happen because they recognize that society's overall interest in developing adults who don't grow up to be transphobic shitbag whiners is more important than who wins or loses a stupid child's game.
Do you know what is actually fair? Leagues making participation decisions based on all relevant societal interests, rather than just your transphobic ones.