It's because Kullervo can appear in any run on the right moods. Meaning that if you're farming pathos clamps for incarnon adapters and Acrithis's evergreen loot you'll get some Kullevero's Bane just through bounty RNG.

1. Do the quest first. You might want to farm a basic amp if you haven't done Chains and the Harrow and The Sacrifice yet, since those are harder with a mote amp.
   2.The META weapons vary depending on activity, but Incarnon weapons are currently the strongest.
2. Very much depends on role and mission. Each frame excels in its own way. But right Now Revenant is best for just not dying no matter what is thrown at you. Wisp is the best general support frames for stationary missions. Saryn is still the best high enemy density endurance nuker and that's unlikely to change. Trinity is still the best source of energy for the whole squad.