[SDPhantom]

RegisterAttributeDriver() is just handling what unit is selected when you click on the frame and should handle RegisterUnitWatch() fine on its own. It's just a more direct approach to what you were already doing. The display code, you can do whatever you want and it isn't going to hurt the SecureUnitFrame. The problem is still located there. Try taking the code back to the basics and replace the 4 vehicle event blocks with this. Don't forget to unregister UNIT_ENTERING_VEHICLE and UNIT_EXITED_VEHICLE.

function Player:UNIT_ENTERED_VEHICLE(unit, showVehicleUI, arg3, vehicleType, vehicleID, arg6, vehicleGUID, arg8, arg9)
    if showVehicleUI then
        self.frame.unit = "player"
        self:UpdateDisplay()
    end
end
 
function Player:UNIT_EXITING_VEHICLE()
    if self.frame.unit ~= "pet" then
        self.frame.unit = "pet"
        self:UpdateDisplay()
    end
end

(Code taken from Player.lua and modified)