Ya know -- that hand truck wheel gave me an idea that may be helpful to the OP. Rather than building a typical single-axle trailer, you could build more of a "wagon" for transporting multiple adults. Think of a gardening wagon with four of those pneumatic tires suggested above, with the front axle being steerable. That would completely eliminate the problem of proper weight distribution, and would remove all weight carried by the bike itself. You'd still have the issue of drivetrain strain, braking capacity, and bike frame torque if your hitch is not right at the rear axle, but you would add a margin of safety with a "wagon" instead of a "trailer".