For the archon I would either-
keep splinter pistol, drop agoniser for electrocorrosive whip, and keep soul trap
or
drop soul trap and keep agoniser + splinter pistol and pick up combat drugs.
or
drop all of the above, keep splinter pistol, pickup venom blade and take a phantasm grenade launcher.
last one sounds crazy I know

but since you are putting archon with Incubi and incubi do not have grenades so will not get to strike at initiative if they charge anything thats in cover, or counts as being in cover. That and with all the incubis power attacks losing power weapon on your archon isnt so bad. The venom blade will also make your unit a larger threat to high toughness things.
the electrocorrosive whip is often neglected, its pretty awesome. You are most likely striking first, so if you are in a challenge with a character or fighting a monstrous creature, and you strike first and cause any unsaved wounds you half the models strength. So a model with a powerfist that is strength 4 becomes strength 2, for a total of strength 4 with the powerfist. Most monstrous creatures will be reduced to strength 4 or less. This means most models lose the ability to cause instant death to you from double strength versus your toughness, should you fail a shadowfield save. Also as it counts as a power weapon if you kill certain things your strength doubling from soul trap means your now Str6
considering your army selection I would much rather have my incubi striking at their initiative when I need them to, as that is 12 klaive attacks on the charge versus making the archons 6 attacks on the charge more powerful. I would go with the phantasm grenade launcher. On the charge agoniser as you have loaded out is average 2 wounds against T4, Venom Blade is Average 3 wounds, of course one is AP3 and the other is not negating
AP. Ironically the venom blade is actually better against 2+ save models than the agoniser of course if your with incubi klaives are all
ap 2 so that doesnt matter much..
tldr- I recommend phantasm grenade launcher on your Archon if hes going with incubi so they can strike at initiative when they assault.