and even potentially be a deterrent to entry for most of the women who are don't have the Spark, but there have to be at least a few women in a city that large that are born with the Spark from time to time and they would almost certainly end up in the Tower, odds are there should be at least 1 Tar Valonian Aes Sedai ... even if training might end up being more difficult in the ways you describe (and I agree with that), the odds that no one manages to work around it seem astronomical.

Perhaps it's these Sisters, who we don't actually know the identities to, who end up administering the city. Who better to be on the council of Aes Sedai who run Tar Valon than a few native daughters?