Hm, I didn't have a problem with necklace length with my later-years Nellie.
The dress did seem kind of big for my Nellie when I was putting it on, but I figured it was a PM v. Mattel thing. It still looked beautiful on her though. I always rearrange the way Felicity's dresses hang on Nellie's torso and from her hips to get it to look the way I want despite the PM/Mattel difference. I also added Felicity's undergarments; I forgot what they're called, but they're the ones that fill up the area of the hips. They made this dress look great, and definitely more Christmas ball-like!