You should contact Microsoft Licensing and Pre-Sales Support for this question. Answers provided here will NOT be a valid defense in a Licensing Audit. It is possible the DC that is using Per Server licensing was just an oversight and it should have been converted to per seat licensing. In addition, my understanding (and every comment here should begin with those two words) is that Device licenses require EVERY device that is going to authenticate to your network to have a license - that INCLUDES mobile phones that check e-mail (if the e-mail is handled by Exchange), the HOME USER'S COMPUTER that is used to connect via VPN, and so on.