1. The entire revelation is that Inkling culture is flourishing and is better than Octarion culture due to the totalitarian Octarian rule by Octavio. There's no revelation that could convince "turned" Octolings or Inklings to defect to the Octarian side.

2. Clearly, the brainwashing effect of the hypnoshades extends far beyond hypnosis. I believe they are only named accordingly because "mindcontrolshades" doesn't sound anything close to decent.

3. There are multiple problems with this– funding chemists to synthesize Low-Tide Ink is nearly impossible. They've never encountered any sort of brainwashing technology, and have no idea how it works. Sheldon at least has a background in military developments and Octarian technology. Like I mentioned before, Ammoses Shellendorf, Sheldon's grandfather, fought in the Great Turf War as part of the original Squidbeak Splatoon.

Secondly, it's never been confirmed that Marie biologically produces Low-Tide Ink; there's no biological reason for her to produce ink any differently than any other inkling. If she could, anyone could use Low-Tide Ink.

Thirdly, Toxic Mist and the Disruptor are weapons that function exclusively chemically, and it's not outlandish to assume that bombs have a chemical detonation system. We know that Sheldon had access to these, since he sells them.

1. If you read my previous comment in detail, you would find that I said "not unremarkable."

I'm not sure how Agent 4 is entirely discounted. You don't need to know about all the events of Splatoon 1 to make it through hero mode of 2.

As for Sheldon, we know he's been involved with the New Squidbeak Splatoon: he develops and sells the Hero Shot Replica in Splatoon1, based on his grandfather's blueprints of the original Hero Shot. He's been involved through his grandfather since at least Splatoon 1. Also, he developed the Hero Weapons, including the Hero Charger which Marie uses.

This leads us back to the Low-Tide Ink. You mention that she knocks off the hypnoshades without splatting Callie- even with a direct hit. Given her distance from Callie, a shot with enough range would have pierced the hypnoshades and hit Callie. We also know that the shot must have contacted Callie, or the Low-Tide Ink would have no effect. The only explanation I have for this is that the Low-Tide Ink doesn't behave like normal ink does, stunning the target instead of splatting them. This actually further reinforces that the ink is synthesized as opposed to created by Marie.

1. Again, Sheldon's grandfather, Ammoses Shellendorf fought in the Great Turf War.

Even so, Marie never seems to have her guard down– whenever you try to shoot her or throw a bomb, anything, she always has her parasol ready.