In principal yes, low carb diets outperform all other diets when it comes to short term fat loss in the available literature.
However, if its suitable for your mom depends if she is willing to commit to it. In my experience, low carb is very hard to adhere to for prolonged periods (which is all that counts for health). So if she can continue to live on this specific diet for months and yearts it is suitable, otherwise no
Protein can and will be used for energy and can be converted and stored as fat. However, that process is relatively inefficient so 1 kcal of Protein will be converted into much less than 1 kcal of fat.
It is very hard to create a caloric surplus with protein alone, since its very satiating and foods with a high protein content usually contain high amounts of fat and/or water. But in theory, you could.
You body does not waste calories, the only time when you 'piss out' any significant amount of calories is when you're diabetic, alcoholic of have some sort of renal dysfunction.