This looks pretty cool.
I just want to mention that a lesser known manga converter called Canti has almost all those features (although I'm not sure about the dithering and resizing method), auto splitting landscape 2-page scans except when the two pages form one giant image, trimming, auto cbz creation, colour quantization should be in the next release and much more.
http://www.bitcake.com/?page=project_canti
Edit: Just looked closer at the image and saw what you meant by removal of rows and columns, that's pretty awesome, Canti doesn't do that but it trims white space around the image.
Edit: Now Canti does
http://www.bitcake.com/?entry=entry110322-143300