Couple of questions for you, on User D running Win8 Home; that won't work as only the Pro client versions of W7 & W8 will work on a Server 2012 domain login. RDP is not supported.
Also, are the client workstations physically located at your external company (the ones running the QuickBooks) connected to the 2012 server via VPN? If not, how are they connecting from their location to your location where the 2012 server is? How fast is the connection at that location? Speedtest.net specs would be nice. If this isn't being done, and the external company is using QB 2013 or newer and posting financial transactions to a bank, and you're not using VPN in addition to 2-step verification you'll likely fail external security Audits and the...