You use the trim for level flight adjustments. And if you do the CG calculations and the passengers put you outside the allowed envelope, you do not let them get in the airplane.
Actually, Aft CG is better for efficiency. The tailplane provides Negative Lift to counterbalance the Positive Lift provided by the wing. Moving more weight aft decreases the amount of Negative Lift required to keep the airplane balanced. The bad news is that Aft CG is much more likely to cause the airplane to stall and spin and be unrecoverable.
In some TV program they said that during World War I - in two-seater airplanes - the pilot flying alone had to take a sandbag as ballast. Then the designers changed the pilot's seat and the problem was solved. Can you explain to me what this is about?