is that 70-90 for everyone that's at titan or the total being dropped from the titan loot table?You need to have 8 kills in the raid to complete the quest. You can't complete it if you don't get enough kills.
The main rewards are going from Titan. Mobs drop is just an additional bonus. You need a total of 560. Each Titan gives the random number from 70 to 90. That's for the top title. 7 Titan's is enough to get required quantity. If you have less, you can go and kill extra rift mobs for extra drops.
As it was mentioned in the main post of this thread 4. Event will not be extended, but in the future it will be added again.