i'm more than happy to have a discussion based on any solid ground and more than happy to be shown I'm wrong
i can't really follow your mental math as you have it layed out, so i'm just going to post ACT screens.
Letting Coil tick, casting next coil right as previous fades: 29k (think I jumped the gun on 10th tick of 1st coil, but not a huge diff).
Spam Coil: 28k
I think there is 1 extra tick for the parse where Coil is ticking. I copy-pasted the summary dmg up top onto the Coil breakdown.
I'd say that's just about identical dmg. Done the comparison test now 3rd time with pretty much same results. Letting Coil tick wins, however, because inevitably when spamming Coil it kept coming mid-cast of Rot and there is the .8sec spent on casting and ~.2sec on recovery-reaction dead time before next spell.