Hi, okay I can see that you have been successful adding metadata to most tracks using combination of Naim Import and MusicBrainz data
So I think the issue may be the following
Wav support storing metadata in the INFO and ID3 chunks. The INFO chunk is well supported but only supports very basic set of fields and does not support artwork, the ID3 chunk supports everything but it is not quite as well supported. SongKong writes both chunks, I wonder if the ID3 chunk is causing a problem for your music player, do you know what your car music player is ?
Since you say you can see some artwork when use link to your iPod device it maybe that your music player can see artwork if stored as a separate file in the the same folder as each album but only if the music files are smaller than a particular size. You already have SongKong configure to write artwork to folder if not already there but may be worth reducing Resize artwork if dimensions larger than (pixels) on Album Artwork tab to see if that improves artwork issue. If you could give examples of where there is artwork and it is shown, and where there is artwork but not shown that would be useful.
The other option now that you have used SongKong to get the naim metadata into your files is now convert the files to another format that your car player maybe able to deal with better (and would take up less space)