I guess I didn’t look at the set too closely, because the +1 to Necromancer skills is already on the helmet, lol. The cold converted to poison and additional RR to Decrepify is already the helmet modifier. I had found Corpse Explosion underwhelming at 12/12, even with Bone Spear and Death Nova both proccing it on death, so taking the points from Death Nova and Corpse Explosion will help the skill-point requirements by putting the points in Bone Spirit and overcapping Bone Spear.
I happened to like Death Nova a lot because the large DoT component was really good while positioning for optimal Bone Spear usage. Hopefully the bonuses to Bone Spear / Bone Spirit will outpace the loss of using Death Nova.