I dont think verifying a student is enrolled at a certain grade in their appropriate enrollment period is hard to obtain. just make it a requirement to get from the school along with their birth certificate. I'm sure there are parents in youth soccer that will make sure nobody is forging either.
As far as the 14 month window, that's already the case for colleges recruiting kids across the country within a specific grad year. Kids from one state will be competing with kids from another state that had an earlier enrollment window for scholarships. Those crying about a 2 month variable window are really only worried about their kids spot on their youth soccer roster.
There has to be a line, and 12 months is fair. I am also fine with MLS stay in BY and ECNL goes to SY. But no waiver, no parents trying to find a loophole to their kids can be the oldest and play against other kids 13/14 months younger. If my kid has to be in the B team without a loophole, then be it. I am totally fine with it and that is what I am preparing myself for the possibility after the switch.