Just had a look at Refinery CMS. Very impressive. Definitely a worthy successor to Radiant. Thanks for the great contribution to the open-source community, guys!
We've been working on RefineryCMS for the past several years for our own internal purposes. We've recently released it as an open source project. Currently about 70 sites run on Refinery, and the number's growing steadily. We'd love input from more Rails developers. Take a look at http://www.refinerycms.com, or http://www.refinerycms.org (github repository).
I would recommend Comatose over Radiant. It is very easy to use and integrate.
Comatose is a plugin that you can directly integrate with your Rails application, but it has limited functionality if you compare with Radiant :
It should satisfy all your requirement, mention above
Radiant is really simple for end users there's not much UI clutter to explain.