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Then the arm could slide the magnet upwards to detach, reposition it, and repeat.

This would work, but this is the point where you'd need to involve an external energy source. If you move the truck-magnet-arm system such that the whole system reaches a steady state and the truck has "moved" (relative to an outside observer), to continue any motion, you need to disturb the steady state to reposition the arm and create a new resting point. Doing so would require energy. From a battery, or an engine, or whatever, but you need energy from an external source. And so we're right back to where we started: instead of all this jazz, why not just spend energy to spin the wheels instead?

That could work, yes. You would have to unstick the magnet from the bumper every time -- or alternately just leave it attached and use it as a tow hook -- but it would work

Because the mass is shared. You could get it to move the truck by moving a giant magnet forward but so long as it’s attached to the truck you’re only ever getting out the movement you put in to the magnet.