This lawyer letter goes beyond what DMCA Sec. 512 allows. 512 authorizes requests for "removal of material" that is believed to be infringing; in this case, Sony is demanding suspension of Twitter users' accounts (which even Sony would likely concede are 99.999% non-infringing as measured by number of tweets). The letter also raises non-DMCA CFAA and state law claims.
> Does DMCA cover unpublished copyrighted work?
Yes. The DMCA covers "copyrighted work[s]," and publication is not necessary for copyright to attach. The longer emails likely meet the threshold for copyrightability and the screenplays, even unpublished, are the very definition of a copyrighted work. Note fair use still applies to copyrighted works.
> If the DMCA applies to trade secrets and personal email
The C in DMCA stands for "copyright." It doesn't apply to trade secrets.
David Boies, the lawyer who wrote this on behalf of Sony, is a very capable hired gun (he represented DOJ in its futile pursuit of Microsoft and plenty of other high-profile plaintiffs). Twitter is going to have to be careful in how it responds.
Remember news organizations have also reproduced excerpts from the Sony-hacked emails, and in some cases the entire correspondence. Note Sony hasn't threatened to sue them, at least so far. Also note Boies' letter could have been addressed to the individual Twitter users, who are actually the ones responsible for posting the material. It wasn't.