For questions 1-4:
The Text (.txt) files are the ones that truly matter and must be present, as they are the only thing between the two that get loaded into the game.
The Excel files can be used to generate the .txt files via a macro, and organize the data into a format more easily viewable by the modder. That being said, they have 0 direct impact upon the game. The .txt files can be created directly if you use the correct formatting.
For question 5, I would like to refer you to my Main Campaign Modding thread stickied at the top of this forum. In there I have a few examples of Stats entries and what each line means.
