Weird, I found it really easy to make Cass my Divine.
My first PT I got Leliana because I did nothing about the Divine -- I also did not recruit Viv on that pt because I didn't understand how the war table worked yet XD I know I told both Cass and Lel they'd make a good Divine, I didn't do any WT operations, and my character was a mage so she was very supportive of mage freedom.
My second PT I wanted Cass as Divine, and I did recruit Viv. I was friendly with Viv and she eventually brings up the point of supporting Cass as Divine and I said yes. Also, I did a WT operation to support Cass, and I talked to her a ton about being Divine, saying she would be awesome, and on her Seeker quest I told her things like "You can do whatever you set your mind to, you're awesome!" I also told Leliana to forget about it when she talked about being Divine.
I would never want Viv as Divine. Man that sounds terrible. I honestly think I like Leliana as Divine, but softened so it's not bloody.