The reforging of Narsil used *magic* and is therefore excusable. I've
made a knife in a blacksmith's forge before and know the process involved in my bones- but that scene didn't bother me because it was so far from reality as to be magic. So obviously, the elves used some magic in the reforging.
And so they should!
As for Shadowfax's "stunt double"... well with the humans they were always careful not to show the faces of the doubles. The
first time I saw TTT and Gandalf and Aragorn were riding up at the end and talking, it utterly ruined the moment for me because I noticed immediately that it was a different horse. Horses have faces- that's all there is to it- and for some reason they thought any old grey horse would do.
The other horsie thing that bothered me were the Black Rider's mounts. Why in the world would someone shoe a horse with the nail coming so high in the hoof? You risk laming the horse. And not clinching the nail down means the shoe wouldn't stay on. And if the naiil is not part of holding the shoe on, and is some sort of offensive weapon(?), then it was a bad idea because the horse seemed to be gashing his opposite legs with the nails from all the blood that was running down it's leg.
A *perfect* thing in the movie was when Bilbo accused Gandalf of wanting the ring for himself and Gandalf got bigger and the walls of Bad End seemed to flex a bit. THAT was cool.
Very appropriate, I thought. Kind of like Neo in the Matrix when he learns he can control everything.
edit: not that I mean that walls flexing means the same thing other than showing an excess of power available in the individuals involved.
Like reforging Narsil with me - I don't know a lot about sword-making, but I can see so many things wrong with that scene that I quite forget to enjoy it. 
Me too! Great point, Voronwë!
