PKW and I have had a looooot of conversations about NC Sundance, and what that means in terms of her manufacture.
I don't know if I've seen one actually stamped China before, she might be like Chinese Majesty, she exists but isn't that common (but also not that expensive). I genuinely am not sure about that.
While China Majesty can be explained by later European releases, however, I think that the wrap skirt M&S issue is unique to the UK. I think most European countries had Megan with the dress, even if they had a NC sundance. (I may be wrong about this though).
I'm not sure how we read NC Sundance, if we read them as made in China or Spain or what. The UK box for 1986, which is the year with the wrap skirt, says Made in China, so it doesn't surprise me that there are some stamped that. It's an assumption that all such NC ponies are Spain but I am not 100% that Sundance is. Also, the existence of the Princess Sundance/Sunpearl variant, which seems to have come from the same put together job factory as the other mix and match princess variants, would suggest they are China. All the parts used in frankenstein princesses and such are made in China I *think*.
The wrap skirt thing is interesting. I wouldn't be surprised to find multiple factories making M&S sets at the same time. But I don't really have any more to add to this.