For these q's - I would suggest reading Lyle Mcdonalds books - he really goes into excruciating detail about protein sparing, glucose, etc. Even many free articles on his site.
From what I know - the brain, some tissues, etc. only like to use glucose. They don't use fatty acids. Therefore if you go low carb - you will NEED protein. If enough protein is not present - it will go to the muscles.
The reason why it's essential to get enough protein when you go very low calorie.
and yes, high intensity workouts can only used stored glucose for it's energy. In a low calorie diet - you are very low on glucose in the morning, so high intensity really is out. You can try but you will bonk out fast.
Which is okay - weight loss is 80% diet anyways. Many like to do relaxing low intensity or weights. Which is good for it conditiosn them to continue doing them in the maintenance phase, which is vital - for most people get fat because they simply go back to their old habits. Overeating and not working out.