Right, you said right in your first post that you were taergetting older devices, and I completely overlooked it. Sorry. Everything I chimed in so far can be disregarded,, it's been a long time since I worked with these, and I'll just get my memories jumbled trying.
As for loading manga on a mini...
Image loading in non-book formats, (cbz) used to be terrible in older kobo firmware, slow and poor quality dithering. it was improved immensely in more recent firmware, but I forget if it was late in the 3.x series or the 4.x series.