Now, I'm not a Summoner, and Shadowflare may have different mechanics from my scholar to my summoner. However, so long as something is inside Shadowflare, I have noticed that its slow effect is always up. I actually use it for damage mitigation/damage during regular dungeon runs because... slow seems like a pretty good debuff.

Miasma II is a soloing spell primarily. You shouldn't be in melee range in any fight where you actually need to use a maximized rotation, so you should probably drop it from this theorycrafting conversation.