"Should only one person in fleet keep their eyes open?" - while it helps to know at least one person is paying attention, everyone could and should use dscan and look for incoming threats and targets.
14.something AU is the maximum dscan range, skills do not affect it.
Sometimes it may be useful to have different people check different dscan settings, for example have one person check the 1 AU range for ships that warp in to your ESS while the others can keep track of the long range dscan.
I make it a habit to find ships in space whenever I fly through that space, for example while exploring. See an Ishtar on dscan? Find the anomaly. See a mining fleet? Check which moon/athanor they’re on. Not that I need to find them, I’m just passing by looking for relic sites, but it’s nice practice.