While playing the demo helicopter single-player mission, I decided to go inside the passenger cabin to see what extra gear I can equip. During my gearing up, the assault team (aka, passengers) were boarding the heli.
After I grabbed my extra gear, I scrolled for an option to change seats from Passenger to Pilot. No Pilot option was available. So I got out of the heli and moved next to the cockpit. There was still no Pilot option. And one of the passenger is sitting in the Pilot seat.
There is no way to enter the heli as the Pilot if an AI occupies that seat.
So I restarted the mission and let the whole assault team enter first. Then I thought to try if I can still enter the passenger compartment. No deal. I can only enter the empty seat (Pilot).
The player should be given the option to enter, swap, leave any seat in any vehicle.