One issue with the experiment is it has such a narrow use case. Disappearing after one download / 24hrs makes sending a file to multiple people--or just one person who drags their feet on the DL-- makes it really inconvenient to use. Even offering "1 download -OR- 24hrs" would make it far more useful.
There are other existing solutions like https://share.riseup.net which do not do things like this too and which do allow multiple downloads.
Do they communicate that sending any known file is non-private?
If I email myself a pop-song, does the client automatically start the extradition proceedings for the RIAA; or does that come later? /s
Also, you wouldn't need add a file. Zipping it alone would be enough to change the hash.
I don't think you can expect the file mod time to be identical, as it isn't preserved through many forms of file distribution.
Why would they do that? It's not like anyone can send them DMCA notices: the act of verifying that the upload violates your copyright already removes the file from the service. And I don't see anything in the DMCA that requires them to search for offenders, they just have to act once they become aware?
The Megaupload case sadly is a good example of what no one wants to repeat, as there this was exactly argued - that, once notified, the hoster has to ensure this file won't ever be uploaded again, and all other copies are removed, too.