I think that a "Grey Jedi" under the Morality mechanic, i.e. one that uses the dark side without succumbing to it, would require the players to ultimately "game the system" to some extent, and simply try to limit how much conflict they gain when using powers like Unleash, Harm, or even Bind (the "deal damage if you used DS" to activate aspect) so that when the session ends, they don't take a major plummet on their Morality score due to a low roll on the d10.
As for Luke, that was in the wake of Jacen's "Unified Force" theory from the NJO series, and said theory was utlimately broven to be a load of bull, with Vergere revealed to have actually been a student of the Sith and using false teachings to push Jacen down the path of the dark side to become the Sith Lord that she and Lumiya felt the galaxy needed. YMMV on the quality of that particuarly story arc (lots of folks didn't care for it how ruined Jacen in contrast to his pre-NJO portrayals), but it does put a pretty big nail in the idea of a "Gray Jedi" being able to use light and dark side Force powers without penalty.