Erin Brockovich style success is very rare.
A game plan is as follows:
1. Get it working
2. Market your awesome idea, start making serious money
3. Identify your billion dollar company's arch enemy / competitor or similar who can set lawyers upon your billion dollar company and win.
4. Sell the running and growing business to them
5. Send me 10% for this great piece of advice :-)
What do you mean when you say "I want to make sure that whatever I have done [...] remains mine"? Are you writing code? If so, it is difficult to imagine a scenario where you wouldn't you retain the rights to your own code. (Naturally, that doesn't prevent anyone else from implementing the same idea).
Clearly, you need to make sure your business plan can survive changes from the big company-- if your idea is that dependent upon what sounds like a loophole in their TOS, you're odds of success are slim indeed. How do you know they don't have a team working on an identical system now, ready to launch next week?
Get a lawyer, have him review the contract, and ask him to advise you.