how to grant the following from our servers?

If this is a trusted Vendor, get them VPN access with a keyfob(secure ID) to access your systems. The secure ID will require them to create a pin so not just anyone can access your network, if this is at all possible. If not I would wait for another expert to chime in.

Personally, I'd be asking why they need that level of access. What service is the vendor providing? Also, do they need to have this for stuff they have to do manually, or do they just need to have a service account with this level of access? We need a little more information to properly answer the question.

If you trust him you can create another VM with Windows 7 / 8 / 10
Create user for hime and add this user to Domain admins group.

Enable Audit on domain to know all changes and give him RDP or VPN access to this VM station.

On VM station you can install Microsoft Remote Desktop Manager and configure access to both servers , Exchange and DC

I'm not understand why he need access to all mailboxes, you need to discuss this with your Boss, since some company information, specially his can be classified.
If your boss will be OK with this then you need to give this new user full success permission to all mailboxes in EAC or using power shell

You can do this by adding user to Organization Manager group

Add-RoleGroupMember "Organization Management" -Member "<account name>"

then

Get-Mailbox -ResultSize unlimited -Filter {(RecipientTypeDetails -eq 'UserMailbox') -and (Alias -ne 'Admin')} | Add-MailboxPermission -User admin@example.com -AccessRights fullaccess -InheritanceType all -AutoMapping:$false

So My advise is adequate

Would I create the server in the active directory or use the wizards in the SERVER 2012 application?

Regular new VM installation would be enough.
You can delete this workstation after all will be done if you don't going to need it.

After you setup new VM workstation with new account install Microsoft Remote Desktop Manager and add this servers you want to give him access to. Test connections.

https://www.microsoft.com/en-us/download/details.aspx?id=44989


You don;t need to create VPN if you don;t want to. Just enable (reroute) RDP on your firewall to this new virtual machine

I have a filling you're not reading my comments.
I just explained you everything.
You can't give access to 2 machines in same time from outside because only one rule can be created on firewall.
You can change port for RDP to one server but it's a lot more complicated.

So best solution is give him RDP access to one of workstation in your domain then setup RDP using MRDM to servers he need. If you don;t have spare workstation you can install Virtual Workstation on your Virtual server and setup it for him. I did explained this in my previous posts.

Watch this video it helps you setup up what you want for a home PC but its the same principal.

http://www.appdataworks.com/allow-remote-desktop-connections-from-outside-your-network/

You need to create rule on your company firewall to forward port 3389 to internal IP (workstation or VM)
It's very simple.

You can use the Public Server Wizard to open up a port (most user friendly way to go about it). The question is whether you want to open port 3389 or use a different port.

Wizard is on the top of this screen after log on

If you really wanted to know the best practice involving RDP, it would actually be to use the Remote Gateway and other services. But for how you actually have it set up, the most ideal things to do would be to have two factor authentication using a product like Duo or AuthLite.

No, wizard will help you open RDP ports and forward protocols to Host you want to give access to.
You can define host before wizard or do it as a part of wizard :)

It's two pieces: The server has to be configured to allow RDP connections, and the Sonicwall has to be configured to forward the ports necessary for the RDP connection to the correct server(s).

Correct. That's where the Public Server Wizard comes into play, to assist you with port forwarding and NAT rules.

Like I said before, you can't configure multiple connection using same protocol and same port number to multiple devices.
If you want to connect to multiple devices from outside then you must configure VPN connection first then connect by local IP address.