Interesting thread. Couple of things....rabbits should neither gain nor lose weight fast. It is damaging to their liver, especially coming down. Rabbit metabolize fat differently than any other animal and that happens in the liver. Like cats, they are quite prone to hepatic lipidosis...aka fatty liver disease.
As far as protein, in nature, a rabbit eats mainly grasses, leaves and bark if there is nothing else (like in winter). Since domestics were once wild, they havea similar need. Any pellet contains far too much protein for a proper diet. A rabbit doesn't really need that much protein in their diet....assuming they are being fed properly. In our rescue, we have some rabbits that eat only hay....now the key to that is having very high quality hay. Evenour 30 pound Flemish gets about 1/4 per day. We use pellets only to maintain weight and body conditioning. I have no concerns at all if they never eat pellets....as long as they empty their hay racks.
The way to lose weight in rabbits is the same as for any other species....calories. Lower the intake and increase the burn. I would start a weight loss program by reducing the pellets by about 10% every 4 or 5 days....maybe down to nothing over a period of several weeks...or no more than 1/8 cup per day if you are offering pellets. Unlimited grass hays....and maybe increase the greens a bit. Monitor hydration carefully since hydration is so important to liver and renal function. Increase the exercise as much as possible. We had a mini lop that the people thought was a French and couldn't understand why she would not gain weight. Well, this mini was a maxi and came in at nearly 10 pounds. It took us over a year to bring her weight down. Today, she is slim, trim and healthy. Point is...have a plan and do it gradually...don't rush it.
Randy