From what I can understand from my (less than perfect) 古文 reading of the introduction to this book on google, 苍润 is an adjective to describe a work of calligraphy (苍劲滋润）which is both vigorous and fluid, but 苍润轩 was a particular place named as such by the painter 文徵明 because of its visual appeal. As such it could be taken as a proper name and simply rendered 'Cangrun Pavilion'.
Alternatively if you wanted to translate the meaning of 苍润, I might suggest 'Proud and Vivacious Pavilion'; 'Bold and Flowing Pavilion'; 'Grand and Fluent Pavilion' or somesuch.

To be honest I think the meaning is so difficult to encapsulate in a few words that I would tend towards leaving Cangrun in Pinyin, and since the origin of the name appears to be addressed in the work itself rely on that to communicate the meaning.

When it comes to 碑跋, I think it is the steles which are contained within the pavilion rather than the book which was written in the pavilion, but I have only read a little of the book so I can't be certain.