Yes, this concept has been explained pretty clearly. For example, if Braska had decided not to challenge Sin, then the Fayth created from Jecht would have been able to impart Braska's Final Aeon to any summoner.

I envision the path that a Summoner must take to get all of the summons like a racetrack. I am certain that Dona or other summoners had access to other paths and transportation that wasn't available to Yuna for a trip to Besaid if they wished to receive Valefor.