Frankly, I don't think you can overcook red lentils - they should be mushy in my opinion, so I don't really understand what you mean. To me if they are even slightly hard, they are not cooked properly. Same goes for green lentils, which have to be soaked overnight, rinsed and boiled in fresh water.
I don't soak red lentils.
Obviously, if you like them mushy as I do, you have to take care they don't boil dry! I bring mine to the boil in plenty of salted water, turn the heat right down and let them simmer until soft under a tight fitting lid.
But I would not have thought there was any dietery advantage in using lentils instead of meat. And surely the starch content is not good for a diabetic?