Can I use a different embassy location?

I live in Michigan but have a niece living in Washington DC. Can she submit all my documents to the DC office for me?
Also, we are using a tour company in China. I have listed the hotels on my application. I am also providing my airline tickets, and tour itinerary with my application, passports, and photos. Is there anything else I need. I have tried calling both the DC and Chicago office and no one picks up the phone during your stated business hours.
Thank you,