A FOC (Forward only conversion) removes the reverse gears from the transmission as well as the linkage. There should be a little blue plug where the shift linkage went into the transmission. If there's no reverse gears there's no need for the Optidrive module or shift servo. Some 3.3 models of are o actually came without reverse. If you really want reverse I recommend buying a transmission that already has reverse in it as well as the linkage and such from an eBay chop shop where they part out brand new vehicles.
Yes the splitter/y-harness is a very common thing. It doesn't have to be a Traxxas brand but the Traxxas one is pretty small and easier to stuff into the radio box, especially when the Optidrive is present.