I'd not worry about having to resort to feeder fish for a short time. The LSN just cheated death and is still not out of the woods yet - if he eats something, I'd give it to him all he wants.
I think, if the filtration can handle the LSN all of a sudden (unlikely though) eating all the rosies, IDK, a dozen or two of them, I'd leave him with a bunch of rosies and just ask your brother to observe. It is a real bad timing right now for you to leave when you appear to be so close to the success, so your brother will have to be your eyes.
Lovely dane! So bad their lifespan averages ~7 years. My neighbor has one completely black right now, 2 years old, easily 200 lbs, and has had many of them over 30 years.