I agree that there should be more difficulties for missions than just the base difficulty and the 240 light ones, and the reason for the Daily Heroic Story mission's keeping you "locked out" is the same as the Nightfall being "locked away." They're both "Endgame Content" now; they both require TTK light score, and yield TTK level rewards. I am confused by your third point. The rewards are still there for Year One activities.