It has to do with the way that waves propagate through any medium.

The electromagnetic waves that travel down a conductor represent changes in the electromagnetic field across the charged particles. This does move the charge carriers. But it is the EM waves that are essentially transferring the energy.

For example, the rare at which the electricity travels down a copper wire is just about the speed of light. But the electrons themselves don't move that fast down the wire. They are accelerated back and forth and do drift down the conductor but the particles arent moving down the conductor at the speed of light. They move at what is called the drift velocity. Which is lower than it would take for you to walk. But that's because the energy is transferred via electromagnetic waves. It's not like a faucet or water where electrons flow like water.

And how you define how it flows in one direction depends on the scale. It does go back and forth but you can't violate the first law of thermodynamics. Power is energy over time. (Watts = Joules per second).

You can't have a passive load (the thing there needs electricity) sending energy back towards the generator (the thing that is making electricity) without violating the first law of thermodynamics. The one but caveat there is of course that the energy can be reflected back when it hits the load. That happens when the impedances don't match, which affects the power factor (which for the mathematically inclined is cosine of the phase angle between voltage and current.)