1- Using HIIT type stuff for conditioning depends on what kind of conditioning you're after. If it's an intermittent sport like say football, then HIIT type stuff should be the primary if not the only method for conditioning. Something like combat sports or basketball would need a blend of HIIT stuff and longer duration as well
2- If you're going to do HIIT type conditioning seriously, then some carbohydrates are needed in the diet, or else performance is going to be inhibited. It's my opinion that it isn't really necessary that the carbs come before the workout, just that they are included in the diet as a whole.
You can still eat "low carb", but you're going to require some carbs if this type of training is taken seriously. Just include some carbs here or there. This is mostly to ensure that glycogen levels can be restored from session to session.
3- You will definitely see gains in conditioning from this kind of training, and if diet is in order you will see fat loss as well. HIIT type conditioning has been proven to be effective for fat loss just as well or better than longer duration conditioning