On the technical side, are high-end dash-mounted units dual or single freq? Can they access PD corrective signals?

Most hand-helds provide 2-15 meter accuracy, unless DoD decides to degrade the signal (Selective Availibility--SA). To correct for this Selective Availibility various commercial outfits and USCG (near ports and waterways) offer signals that can largely remove the error introduced by SA. If not available, then the ol' GPS could be degraded by 10's of meters.

If I were an airplane pilot or maritime navigator, I'd not become reliant solely on GPS for navigation.

Keep the ol' sextant and chrono handy, boys and girls.