Why do you need an opto at all in your LED driver? There aren't any high voltages that justify one. If the bottom of R8 is taken to ground instead of to the 12V rail the opto can be omitted and the circuit can be simplified considerably.
you mean make it a boost converter instead of buckboost?
...not possible as some of the loads that may be attached are less voltage than the vin that they may be used with.