I don;t think that Toph will ever be able to refine how power to that point. The metal in a human body (or any body, for that matter) is not in sizes large enough for even Toph to bend. She needs to actually be able to sense what she's bending, and she requires contact with the Earth/Metal to bend it.

Most of those have already been done. I think that all metals come under the catagory of "metal-bending". Aang and the Dai Lee had no trouble bending crystals in COD. I don't think that glass would be bendable though. I think that that has been taken to a stage where it really, really isn't "earth" anymore. It's like wood - one of the materials in the Avatarverse that cannot be manipulated by benders.

I agree wtih you on Toph being the only one who can learn metalbending in a few hours, but that doesn't make her more powerfull at regular bending. I know she has a connection to the ground, and knows original earthbending. But I'm going to give you a clear example now:

I'm in my karate class. We are going to learn a new technique. It's not realy a karate move, but our teacher wanted to teach it to us because it is very usefull for self defence It's a complicated technique to grab someones wrist and than twist it and put it behind someone's back to disable him and hurt him is you need to. I have a blue belt, and my friend has a brown belt with 2 black stripes. So he's nearly a black belt. I learned the technique way quiker than him. It took me only minutes to do it properly while my friend took about and half hour to do so. This was also the case with some other kicks and puches we do. I learned them quiker than him. But when we went sparring, we could use al the techniques we learned there. I got my ass kicked by the guy, simply because he is better at the basic techniques, and because he has more experience. So my point is: if you can learn an advanced technique much quiker than someone else doesn't make you better at karate, or earthbending in Toph's case. So she isn't the best at regular earthbending.

Metalbending is a very advanced technique, and I don't think someone like Haru or Boulder could ever learn it, but I'm convinced that Bumi can. He also has a connection to the ground because he could sence firebenders from a very long distance and than take them out with earthbending, as seen in his takeover from Omashu. He could also bend house miles away from him, that he posibly couldn't even see from where he was standing. So I don't think Toph is the only one capable of learning metalbending. She has the most raw talent of any earthbender in the show, maybe more than even Azula or Katara. But as I said, having more raw talent and being able to learn moves quiker doesn't make you stronger, and the only one capable of doing it.