I wonder if #4 could be (sort-of) implemented as a very long-term loan? College loans in the US can have a lot of those features, they're just not income-adjusted.

Another way to profit from this is spreading ideas to the students - when someone spends 10 years in a boarding school, they're going to be very influenced by what other people in the school think. It would be really dark-artsy to go all-out in indoctrinating the students into your values, but they're bound to absorb some things from their teachers unless you intentionally try to prevent it.

I think one of the difficult things would be identifying the gifted children. You might a lot of parents applying "just in case", and it would be a balance between that and missing a lot of gifted children because they didn't know about the program or couldn't pass a barrier like an application fee. And if you're recruiting from extremely poor populations, you'll want to take children in as young as possible so they don't spend too long on an insufficient diet, so you might have to find an intelligence test/filter that works for children who can't read yet.

Overall, I like this idea very much. It could make for interesting meta-charity, too.