1. The current flow alternates between flowing down one wire then the other. It doesn't flow down each wire at the same time.
Another analogy: Imagine standing at train platform and in front of you is a circular railway track with a train on it that is so long it connects with itself.
If the carriages moved to your left, more carriages would be pushed/pulled around from your right to replace the ones that just moved to the left.
If they reversed direction the same thing would happen but now carriages would appear from your left.
Repeat this oscillation 60 times / sec & that's a 60 Hz cycle. Note that the carriage's movement (ie the electrical current) is only in one direction at a time. In one end, out the other, change around, back into the end just left & out of the end originally entered.
2. You will feel a shock if you complete the circuit and you are also the path of least resistance to for the current to flow through.
Your body isn't able to tell the difference between the current arriving at your left hand / leaving by your right hand or visa versa.
If you connected a meter with a polarity that caused a centred needle to deflect one way or another, depending on the direction of current flow, then you would see it flick from L to R and back again.
3. Repeatedly reverse the water flow & it describes AC. Keep the water flowing in one direction only & it describes DC.