Each difficulty you go up gives you a 15% higher drop chance. If all you're looking for is legendaries, then only increase your difficulty if it doesn't lower your GPH by 15% or more. Since the bot can end up doing a lot of needless backtracking and the like, sometimes the increase in...
The safety net is actually ten hours and it starts kicking in at six hours by slowly ramping up your drop rate until it reaches 100% at ten hours. They said their goal with the base drop rate was for people to go no more than two hours without a legendary is all.
As for the event, I wouldn't...