The only real test is to make it open source. There isn't a lot of money in cryto done correctly, that is closed source. Because without public audits its impossible to know you've done it correctly, and even if you have, the public perception will be you haven't due to its closed-source-something-to-hide nature.
Makes sense to me. I've always wanted to get involved in a cool open source project. Anyone want to get this started? Make it a free time activity or something? Or are there already better solutions out there? No need to reinvent the wheel.
Not in this direct line of software of the encrypt multiple documents and only out 1 based on pass-phrase, this would be unique (as far as I can tell).
True crypt tells you how to set this up, but not do this automatically. Also it would require you selecting which volume, not just "insert password get document"
This would offer a higher degree of plausible-deniablity, and portability, by making it a file its not tied to one location. The structure of the file, and multiple hashes also grant plausable deniability why you can't just decrypt the entire file in one go and compare the 2 documents SHA hashes.
Would it stand up in court? No. It would help avoid less tech savvy people.