Yes, you can surely use whey protein to complete your keto diet. However, most protein products these days are full of sugars and other carbs. Therefore, you would need to make sure that your protein is very clean, and you will need to study carefully its ingredient list. Such protein as the Isolation by N2BM is a good choice, as it is made with whey Isolate only (whey concentrate does have lactose and consequently carbs), there are no sweeteners, and no unnecessary additives. So it would work very well in a keto diet.
As for the tuna sandwiches, there is one problem here - the bread, it has carbs, and there is no place for carbs in a keto diet.