Isolated stimulator using optocoupler
What you need, instead of a demultiplexer, is a micro controller with 8 output pins. You can then program the individual outputs to turn on one at a time or all at the same time, and any other combination in between. 

There are cheap, simple, micro controllers readily available.  There are ARDUINO kits from $10 to $50 on eBay, with all kinds of online help for beginners (another micro is the Raspberry Pi).

I did not have time to answer your other questions.  So here they are.

You do not mention the voltage you are applying.  The safe thing to do is assume that the cranial resistance is zero.   since you need to limit the current to 200uA, if you use a voltage source of 2v, then you need a 10K limiting resistor (R = V/A).  So, using a 7.6k is a little on the low side, so definitely don't use the 1.8K.
As far as the NOT logic, it does not apply.  Each opto is just connecting a side of the supply.
Note:  Your connections diagram is wrong.  One side of the emitter resistor should go to the emitter, but the other should be connected to one probe and the emitter resistor of its pair, should be connected to the other probe.  Because of this, each emitter resistors should only be 1/2 of the total (7.6K), or 3.8K.