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.

Yes and Yes
Saryn Prime is worth getting, but she’s not necessary.

You want initial combo count so you don’t have to worry about building combo. Easiest way to get it is with the mod Corrupted Charge. Use it instead of Reflex Coil. It’ll give you a 2x multiplier on all your heavy attacks without needing to build combo. Try to get Amalgam Organ Shatter. You want viral procs, so replace your 90% elementals with 60/60s for more status chance.