Margarine will not impact the texture, and probably will not alter the end result dramatically. It, of course, is not butter, so as long as the flavor is fine with you. But, you also don't need butter or margarine. You can use olive oil. Sweat garlic in olive oil. Proceed from that point.
Rice starch is the usual substitute and of course is gluten free which seems important these days. Rice starch IS used in other parts of Asia I'm told to make a similar product. I've given it a try and I'd be pushed to tell the difference. But like anything else it's all about trial, error and experience I suppose.
I think red lentils are just the insides of green/brown lentils. Without the seed coat, they disintegrate very quickly. That’s desirable for some applications but I think it would be a little unsatisfying in a mujadara where I enjoy the textural contrast between softer rice and more toothsome lentils.
You can take advantage of the Leidenfrost effect to minimise the use of fats.
The Leidenfrost effect is a physical phenomenon in which a liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this 'repulsive force', a droplet hovers ...