- No, water is his weakness, it's been established since the first movie.
- The idea has always been to depict Mr. Glass as a genius mastermind. He's not just a bomber, he also needs to have knowledge about civil engineering to plan for the building fire, electrical and mechanical engineering for the train crash; all of those happened on the first film is proof that not only he's able to plan it, but also make it look like an accident. The "conspiracy theorist" part is still there in this movie.
- It's not affection, it's a sense of camaraderie between people who share the same fate. But yes, I also think the relationship is a bit forced, but no retcon there.
No, it's never mentioned in "Unbreakable" that water drains his super strength like Kryptonite. All it said was that he drowns faster than a regular person when submersed in water.
No, it's never mentioned or implied that Glass needs "super intellect" to plan terrorist acts in "Unbreakable".
No, they were never physically affectionate in "Split" and there was no "camaraderie" between Casey and the Kevin personality.
No, there was no secret society in "Unbreakable" or "Glass", which is why there is abundant research on people with extra-normal abilities that is available for Glass and the psychologist treating Kevin.