There can be Depletion region in a MOS device. The NMOS itself cannot be in the depletion region.
Actually a positive charge on the gate attracts electrons to the gate. They cannot get to the gate as there is an SiO2 layer (gate oxide) which stops the electrons.
The electrons gather in the P-region (it's an NMOS so the silicon is P+) and fill all the holes in that area of the P-silicon just under the gate.
So the region just below the gate is now N+ (it has many electrons), the rest of the silicon is still P+. In the region between those two, there will be a depletion region just like in a PN junction.
There is no way any current can flow, the gate is isolated so that positive charge is trapped there for ever unless the gate is discharged.
The mirroring of charges means that the total charge is zero. So +x Coulomb charge on the gate means there will be -x Coulomb of electrons will gather at the other side of the gate. If there was no mirroring of charges, for example one electron is missing under the gate, the total charge (gate charge + charge under gate in the silicon) would still be positive, that would attract just one electron and balance out those charges !
The SiO2 layer is just there, it is needed to make a MOS structure. It is not an MOS (or NMOS) device without the Silicon dioxide layer. Without the SiO2, it would not work so don't think about that situation.
