Does the fact Yuna is not listed on any of the GP assignment indicate she won't able to qualify for WC even if she change her mind later and want to participate?
No, she would just have to do a senior B to get a minimum qualifying score...or ask ISU for an exception like Plushenko did for Euros this past season. Plushenko was able to skate at Euros after arguing that the minimum score was meant to restrict low-score skaters, not high-scoring ones...and he would have to skate in the Euros qualifying round anyway... As reigning OGM, Yuna could easily get the same. and I don't think South Korea has a direct entry at Worlds, so Yuna would have to skate a qualfying round too.
of course, rules could change with the upcoming ISU congress, I suppose.