After some fiddling around, I think the logic is somehow broken. I might be wrong, but when mirror is defined (use_mirror=true) and network is configured (no_network is undef) than mirror-based install should take precedense over the cd/dvd, right? Therefore the 50mirror should be renamed to be before 40cdrom and 41cdset (and simple check should be done to skip media checks at all for apt-setup). Is there any reason to maintain the current sequence?
Rgrds,
Jerzy