Some questions about N-channel MOSFETs
The concept of Ron only makes sense where the MOSFET behaves like a resistor, which is in the linear region. In saturation the transistor is modelled by a current source (with a resistor in parallel to account for channel length modulation). So in saturation you wouldn't use something like Rds(ON).  

1) The power is always given by 
$$P = V_{ds} \cdot I_{ds}$$
so it is sufficient to use the correct value for the current. Rds,on is not required but it could be used to determine the current for small values of \$V_{ds}\$.

2) The depletion region does not block the flow of current. It results in a field pointing from the drain to the channel. Electrons that reach the end of the channel get swept to the drain by this field.