Just to update, I levelled my toons to 19 and tried again, and it worked. Mobs were still 23-24, but CoD landed. Mobs stayed 23-24 until the group hit lvl 30, at which point mobs were 33-34. At level 40, again, the mobs jumped up to 43-44. I'm assuming this is the pattern for this mission. Anyway, the macro works great, aside from the occasional time the tank fails to request a new mission. Can just manually request it (/tar Destrea, /say investigate) and, if need be, enter the instance on your AoE toon (/say ready). After that, the macro picks back up, zoning everyone else in and beginning the roundup on the tank.
One more thing I noticed (not a big deal, but still) was that, pretty much every iteration, when the tank requests the new mission, even though the previous task has been removed on all toons, the request fails because "You may not request a new shared task, you already have one." It's not a big deal because when this happens, the tank always tries again, and on the next try, it works fine. I might try adding a little delay in after the removal of the old task and adding the new one; what I'm wondering is if this is also causing the other issue of the tank not requesting the new mission. I think it may be sending the /say investigate too soon, checking that the group has the task, seeing the old task and assuming the request worked, then proceeding to "wait 40 seconds" for the instance, during which time the old task is removed. I'll update if adding a delay fixes this.
NB: I'm on FV server, which is experiencing an exorbitant amount of lag for the last week and a half, which might also be causing these issues.