synchronous buck - nmos current flow direction
As far as I know in nmos the current flow from the drain to the source

That's not correct. Current can flow in either direction in an N channel MOSFET and, when the synchronizing MOSFET is activated, current flows from the right hand terminal of the inductor, through the load and output capacitor and up through the N channel MOSFET back to Vsw.
4 quadrant MOSFET VI curve: -

Image from this website.
I believe that all your numbered questions are based on your initial misconception about MOSFETs.
The MOSFET drain to source connection is just a &quot;resistive&quot; channel - there's no PN junction involved in it at all so it will conduct just as well with a reverse current as a forward current. The only thing limiting full operation in reverse mode (bottom left quadrant) is the parasitic bulk diode whose effect you can see when Vgs is 0 volts - there is little negative conduction up until Vds becomes about -0.7 volts then, beyond that, it's a diode characteristic.