I don't know the answer to your first question, but I have noticed that when I have lots of bread or sweets, I get really dehydrated and need to drink about two litres/quarts of water before I feel better. It feels as if the carbs 'soak up' a lot of water.
The weight stays up for a few days and then usually drops drastically.
This is not a scientific explanation, but just my experience.
Inflammation is the body's response to injury. In the case of carbohydrates, it's the body's response to too much stimulation. Blood vessels will dilate, water is collected to allow faster movement of chemicals between cells, etc. I'm sure there's a detailed process you can research.