1) Legally an idea is not capable of protection by patent, copyright or trademark. This is a principle of free trade and commerce held up by the US Supreme Court. Thus, I am afraid you just have to keep plugging away with your site without any protection of your idea. You can protect the name of your site with a trademark, you can protect your website design with a copyright, but the idea itself is not capable of being protected. Once you actually develop the site, you can patent the process itself, but that does not mean someone cannot come up with some other way to do the same thing.
2) You make the other company sign a non-disclosure/confidentiality agreement and non-compete agreement, which is where they agree not to reveal your idea or use your idea in commerce to compete against you.
No matter what your friend calls this it is a loan and the fact he is giving you 5% of his earnings for 20 years that is a loan to repay your money you are financing for his school.
1) This is a loan agreement and you would need a loan contract.
2) You would only be able to enforce collection by lawsuit if he does not pay unless he has some property he can give you to secure the debt.
3) You need to require submission of his W-2 forms or 1099 forms or tax returns to verify income and you also need to determine if you would be taking 5% of his gross income or net income (after taxes).
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