As for the Yume no Ato live performance, could it be this one? It is very white once it gets going and she is even wearing a white dress.
http://www.dailymotion.com/video/x6i...E3%81%A8_music
Hmm... I don't think I've seen the other video. I'll keep an eye out it, though. But I'm sure some other fans know what you're talking about and will get back to you as soon as possible -- hopefully with a clip. I'm actually really interested in seeing it as well.