Studying the other player videos about the challenge and playing it, I have developed the idea that most of so called bugs (and the swapping item cheese first amongst them) are meant to be ways for different classes to approach the challenge, in the same way as half of the apparently no-so useful items accessible e.g. from Thaenic are a way to use these "bugs" (e.g. who would normally spend time on Resounding Call Cacophony Call to Arms in a normal game? or Necklace of the Harvest Moon /Midnight Wish?)
There are many levels of difficulty, more like many games in this game, and the ultimate is the last one, where you discover the hidden meaning of most cheesy items. It is hard to realize this if you are not actually forced to discover them like the ultimate challenge difficulty does, it is something hard to describe until you really try playing it.
It's true, part of these items hidden powers (like the chill fog paralysis) could still be some not-meant-to-be bug, but I would still be not sure about it, because against some bosses not even that can help much alone in my experience, they are mostly a way to win more easily some middle/high level encounters in less time or to keep Vela alive while you find a way to crack the tough nut. Same reasoning with the double item equipping as a way to e.g use the leaping boots to keep Vela safe in a couple SSS Arena encounters or to sneak past some mobs.
One of the meanings of the challenge is perhaps to give you this better insight on this.
If I ever start again the Ultimate Challenge, it will be also to try and discover the inner meaning of things like the Nebula Pet