Most projects like this fail because they don't practice TDD. They have pages of code and no single proof that it works as expected apart from feedback from people testing it which is prone to errors.
6000 pages of rules is not a problem. You can implement anything but if you don't care to prove you implemented it correctly then that will almost never work.
Proof and test are different things, you seem to be missing them. A proof is a math concept to verify that all cases are handled, a test is a implementation concept to check one particular case is handled the way you think it should be. Both are important because both uncover a different type of bug. (Proofs uncover cases where you didn't think of something, tests uncover cases where you thought of something but didn't get some detail right)
