You bring up some interesting criticisms, so let's talk about them.
I struggle to see how tying your main method of Aetherflow generation to Adloquium is a tax. Your main Aetherflow tools are either superior to Broil III in terms of damage and Chain Stratagem. Effective use of Adloquium then translates to a DPS gain, not a loss. The outliers are Excogitation and Indomitability as using those will result in a DPS loss. These exist for emergencies and can be used with Recitation without their Aetherflow cost just as Excogitation can be right now.
That said, I can understand being apprehensive to the idea that you can't generate Aetherflow without your barrier breaking. This is one of the reasons why I added in the Conversion ability. You can generate faerie gauge freely, and can easily convert some of it into 1 Aetherflow every 30 seconds. Perhaps this should be learned right away as Aetherflow becomes available?
I think there's some confusion about how Spark II and Meltdown work. Spark II is a GCD DPS action. It generates 20 faerie gauge, has the same cast time and recast time as Broil III, but deals less damage. You could spam it in theory, but doing so would be a DPS loss. Instead, you need to think about how much faerie gauge you realistically need. You can generate as much of it as you want at any time, but how much of that is worth the DPS loss? It's all about decisions and trade offs that you need to make and have the power to make. Meltdown is an Aetherflow action. It's OGCD and requires 1 stack of Aetherflow. I'm not really sure how this a conflict with Galvanize when Galvanize is what enables you to use Meltdown. Can you explain?
Would Helix being the same DPS as Broil III invalidate Broil III entirely? I suppose we could raise the potency up to 290, but make the MP cost 600 like how Ruin II used to work. Enables weaving, but isn't meant to be spammed.
Regarding the Emergency Tactics situation, I can being worried about the weakened barriers on Adloquium and Succor being a concern in some ways. I think ultimately it's just a numbers game. If I were actually the developer in charge of SCH, I would be testing these values in a prototype, so I'd have a clearer picture of whether or not these nerfs would be too weak. One reason for it was to make it easier to break Galvanize since that's now the goal for SCH, but maybe it's a little too much. I'll do a little adjustment.
Regarding the faeries...
First, I will say that I had in mind that this build would exist in a world where the pet AI got fixed/improved. Let's be honest, that probably won't happen, but it is just a theorycraft for fun at the end of the day so let's just say for the sake of argument that it does improve.
Overall, the pet hotbar does still exist, though you make a good point on the ability to manually dismiss your pet. Honestly, I forget this option even exists. I was just trying to prepare for if you accidentally get too far from your faerie. Perhaps it's best to remove the CD reset on Eos/Selene dismissing.
With Eos, I understand where you're coming from with Fey Covenant. That was my way of trying to find an alternative to Fey Union that didn't just drain faerie gauge while disabling the faerie, but it might not work that way in practice. We could just have Eos use Fey Union in this spot instead. Thoughts?
With Selene, I don't think the toad debuff is a modeling issue. We already have an example of Imp being used against enemies in PotD/HoH. On the damage reduction, though, maybe the threshold should be a little higher. We could add Chain Stratagem to new Bane, though.
Eventide generating faerie gauge instead of Aetherflow is mainly to give you a way to get faerie gauge fast if you haven't been casting Spark II enough. Because Aetherflow is not dishing out a lot more damage, I wanted to avoid creating too many ways to get it quickly. Remember that you can cast Adloquium on multiple party members to get multiple stacks of Aetherflow right after a raidwide.
Lastly, I agree that Supersession would most likely not happen with Square behind the wheel. I won't say it could never happen. I think the restrictions on its timing are tight enough that it could be seen as not quite BLU level shenanigans, though it's easily the most ambitious idea for a single action. It's tough thinking about a capstone tool at this point though. What else to you add? Another heal? More damage? I don't think it would be game-breaking personally, not with such a tiny duration.
I'll work on some small adjustments based on some of the feedback.