Given my understanding of your situation I recommend that you use the wireless device to connect a device to the calibre library on your iMac. Doing it this way removes the entire iCloud issue.
Have you looked at our FAQ
How do I setup Calibre Companion to connect to calibre? That discusses how to connect to calibre as a wireless device. The basic instructions are brief because the process is simple: turn on the wireless device in calibre then (try to) connect as a wireless device in CC. Almost all of that answer deal with network problems.
As for the cloud connection: CC guides you through the connection process: signing in, choosing the library, and fetching metadata.
If you put your calibre library on an SD card then you can use CC to fetch books from it. See or FAQ
[Android] I have a copy of a calibre library on an SD card. Can CC use it?
The Icarius is (I think) an e-ink device. You might want to check the "Attempt color correction for E-Ink" box in CC's Settings / Other.