Current limit resistors for Optocoupler controlled Mosfet
You should start at the mosfet Q2.  

I think there is no need to drive the mosfet on a low voltage (2V). Always drive the gate at a voltage that is higher than the \$V_{GS}\$ given in the datasheet. A good guidance are the conditions for \$V_{GS}\$ used to specify the \$R_{DS(on)}\$.
When the mosfet is turned on/off at (very) low frequency, just drive it with the highest voltage available. (When driving at higher frequency, you may consider a bit lower voltage to reduce switching losses).
In your case this implies setting R6 to 0 &Omega;.  

UPDATE With updated question, the following does not apply anymore
Note that this also implies R5 becomes superfluous as it is now parallel to R7.

Regarding the optocoupler: don't short the base connection to the emitter, but leave it open, or use a high ohmic resistor to control the sensitivity of the optocoupler.
UPDATE With updated question, this issue is addressed.

In order to get a \$V_{GS}\$ of 5V, (assuming R6 is shorted and R5 is removed), you need a current through the optocouper of only 5V / 10 k&Omega; = 0.5 mA.

Next, check the datasheet to find the minimum CTR: for the 4N25 it is 20%.

This means (in worst case) the optocoupler needs a forward current of 0.5 mA / 20% = 2.5 mA.

According to the datasheet, Fig 3, the forward voltage is about 1.1V.

So, worst case R4 should be equal to:

(5V-1.1V) / 2.5 mA = 1.56 k&Omega;

It may still work with a (bit) higher value as the CTR is typically higher.