Its in CartoCSS which would be converted into XML that Mapnik uses to define maps and styles. Since we have multiple maps (different regions/countries) on the device, I am planning to define styles using normal mapnik approach (Carto -> XML) and then populate it by the server with the maps installed on device. That should allow to render all installed maps at once without the need to select one.

I am planning to take OSM Bright (it seems to be a rather popular style and the base for several others) and adapt it to SQLite import. I am not planning to edit style as such, there are probably better artists out there

As for GUI, there are probably at least 2, https://tilemill-project.github.io/tilemill/ is one of them.

Yes, there seem to be no problem with high PPI. As far as I can see, user would be able to define scaling factor that (s)he likes and its gonna be rendered on-fly with the same stylesheet but correspondingly wider/larger elements (lines, fonts, ...). So, it all fits the bill, as far as I can see.