Controlling Triac w/optocoupler, need some help with optocoupler datasheet
This is an ON/OFF controller. When opto-coupler is ON, the load is "ON"


  Or does the optocoupler switch off a certain number of degrees into
  the cycle?


The load is not switched off/on as a matter of degrees. The load is "on" according to your command to the opto-coupler. As long as your input to opto-coupler is sustained, the load is engaged, (on).

Your "edit" re : "1/3 A is momentary" is correct"