RESOLUTION:
After speaking with MaxCDN several times and receiving some absolutely wonderful advice from them, I noticed a pattern in the images that were or were not being served. (Staring at source code for hours on end and I hadn't noticed this yet?!)
- The images that are replaced properly have the correct domain name: eg. "http://oncecoupled.com/wp-content/uploads/sites/4/2014/02/comments.jpg"
- The images that are causing problems are using the correct sub-domain name: eg. "http://infinite.nu/oncecoupled/wp-content/uploads/sites/4/2014/02/post.jpg"
With this realization, it seemed that domain mapping was *actually* what I needed to look in to. Armed with a new arsenal of search terms, I pretty quickly came across this: http://wordpress.org/support/topic/featured-image-paths-in-mapped-subdomains?replies=9#post-3906844
Which, in short, is the solution! Click over there for the answer to both featured image paths in multisite mapped domains, edit media file urls in multisite mapped domains, and as a result of these, cdn images not being served properly.