Note: IANAL, and I don't work for MS, but I've been dealing with Windows licensing for many years (as a reseller and privately), so I have what I think is a pretty good grasp on it. ;)
It says 2 per processor. Is that 2 user accounts per host per processor?
I believe you are referring to this statement:
"The licensing for Datacenter edition will continue to be processor plus CAL (Client Access License), with each license covering up to two physical processors on a single server."
It means that for every 2 physical/hardware processors in a machine running Windows Server Datacenter, you need 1 Datacenter license.
CALs are a separate license. For CALs, you need one for each user (or device) that will be using that server. The CALs are valid for every Win 2012 server you may have, so for example, if you have 10 users and 50 servers, you only need 10 user CALs.
But also says unlimited?
It says: "enables unlimited instances of Windows Server with each license"
The Datacenter is licensed per physical processor. So if you have 2 physical processors, you need 1 license (as discussed above); if you have 3 or 4 physical processors, then you'd need 2 licenses, etc.
Assuming you have 2 physical processors, then with a single Datacenter license you can run as many instances of Windows Server on those same two processors as you can/want.
So for example, if you have a VM host that has 2 physical processors, and you want to run 10 guests running Windows 2012, then you'd only need a single Windows Server 2012 Datacentre license (plus appropriate device or user CALs).
What are some solutions to guarantee my actions are compliant with the licensing before I proceed?
First, read the other documentation under "Related resources" (on that page you linked). Specifically the Windows Server 2012 R2 licensing guide, and Licensing Microsoft server products in virtual environments documents.
After that, if you are still confused, contact a local authorized MS reseller and get them to help you with figuring out proper licensing. And/or perhaps contact the MS Volume Licensing Service Center support branch local to your country. If they can't answer you, they should be able to at least point you to the right people. :)
Having said all that...
I have MSDN Datacenter and Standard keys for Windows Server 2012 R2 (5ea).
You cannot use MSDN licences in production. They are only for development and testing. So as soon as you use those MSDN licensed products for anything other than to develop and test a solution, you are breaking the licensing agreement.
Also, MSDN licenses are issued to a single person to use.
From MS' MSDN Licensing:
"MSDN subscriptions are licensed on a per-user basis. One person can use the software to design, develop, test, or demonstrate his or her programs on any number of devices."
So if you have an MSDN subscription in your name, you are the only one who can use the software you get from MSDN.
If you have 5 developers working with you on your solution, each one needs an MSDN subscription to be able to use the MSDN-licensed software for development and/or testing.