I don't read comics unfortunately so I can't help in that area. But I do read Japanese manga. I find that the Amazon JP metadata finder works perfectly and finds all the metadata and covers perfectly for those. Now I just need to format them correctly as getting blank pages in between and work out which file extension is best. I prefer iBooks for reading but may use Marvin for the manga if decide to go with CBX/CBR. I tried using the iOS
ComicRack app for comics and that seemed to work quite nicely but they stopped updating it for the iOS.
Can you find your comics on Amazon because I would have thought if you can, this metadata would be available? I don't understand how book and comic files are different, I guess it is the images that Calibre deals with them differently?
Inukami