I would say :So 4 pages later what is the general consensus or did we all reach one? I personally tend to error on the side of overbuild for safety. I don't want 400 gallons of water raging through my house. Just like I wouldn't fly a single engine plane across the Atlantic.
1. Depends on experience and quality of the builder.
2. On your preference personally, I tend to always like less "bowing" on an acrylic aquarium. So for example, I want to get a 180 gallon to 220 , and most recommend 1/2 inch , I prefer 3/4 inch.
3. At 400 gallons , I would go as thick as possible to be built properly, if your building it yourself and have that confidence, go for it.
Again, I am not a builder but just from reading a lot over the years and looking at experiences shared.