You can definitely use CAD over VPN. See the below line from UCCX Design Guide:
Network Address Translation (NAT) and Port Address Translation (PAT) are supported between CAD and the Unified CCX servers as long as the CAD client applications are behind a VPN
UCCX currently supports Remote Agent Over Broadband wherein agents can use their Cisco Unified IP Phone over a broadband internet connection. For this, the enterprise will need to deploy and set up an appliance which supports SSL VPN connectivity. Connectivity between the remote agent and enterprise must be over broadband/SSL VPN.
Refer Design Guide for above mentioned things:
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cust_contact/contact_center/crs/express_11_0/design/guide/UCCX_BK_U3AF2742_00_unified-ccx-design-guide-11.html
Also as mentioned in other post MRA is not directly qualified however you can use Extend and Connect feature that will allow an agent to use any phone while working from home be it a cell phone, any other normal phone etc. Refer Home Agent with Extend and Connect of the above mentioned Design Guide to know more about this feature.
Regards
Deepak