I don't follow your logic.
For option 2 ("FB providing 4 corpses
at once") you seem to be ignoring Merc completely, or at least whether Merc is there or not makes no difference. But of course he does make a difference. There is a first corpse, and that is the one that he deals the most damage to (unless you throw FBs elsewhere). Now after that first corpse gets exploded, does that immediately kill other cows in the standard scenario that you throw FBs at the same target Merc is attacking? Based solely on observations that seems to be the usual case with any decent setup, so that's good.
There is a tiny window between after the first CE triggered but before the corpses it produces hit the ground for the CE chain. Basically the "corpses flying in the air" window. Do you mean to say FB helps in maybe providing a corpse in this window, so that DS doesn't need to wait on the corpses triggered by the first CE to fall? That would make sense to me at least, though I'd strongly assume it's not worth gearing around.
AFAIK DS works like this: It checks every 31 frames whether a corpse is within its awareness radius. If it is, it will trigger a corpse explosion, otherwise it will trigger a lightning bolt to a random target within its awareness radius. DS traps are treated individually so in theory 4 DS can trigger 4 CEs simultaneously, but only if you cast them 31 frames apart.
If you lay down one every 9 frames, they will trigger their actions within 9 frames of each other. That is my understanding at least.
This "gradual CE" is very helpful in enabling Phoenix main switch, because there's a good chance that when Redemption triggers there will be corpses "in the making" rather than being available for Redemption to steal them all at once. I never tested Necro with Phoenix, but wouldn't be surprised if it didn't work simply due to the lack of this "gradual CE" nature.
