Where is this notion coming from that they can’t be corrupted or that doing the Lightforged ritual makes them unable to be warlocks? At this point in the lore we know fully well that Demons and Undead can wield the Light, or that completely insane mortals can wield it too, so why wouldn’t some Lightforged Draenei, over the course of their stay on Azeroth, wouldn’t get the idea that he or she can use the Fel and believe it’s righteous?
Edit: @Moridunum also posted some of these new race+class combo NPCs about a month ago: PTR Spoiler/Discussion Thread (Part 2) - #2224 by Moridunum-argent-dawn
Not sure how many of them actually made it on Live, but the Kul Tiran duo are in-game in Old Town Barracks and the Slaughtered Lamb respectively.
Edit#2: Checked in Orgrimmar and Stormwind, most of them did actually.