I'm not sure if there is an exact English terms expressing this, but the meaning of this word "共―” is;

共布/ともぎれ/ともぬの：　　same fabric....using the same fabric of the garment front side

共ひも/共ベルト：　String, belt, (or belt loops) made using the same fabric of the garment front side

Or "common" fabric (to the garment body) may work.

Another option:

http://www.merriam-webster.com/dictionary/self-belt
"Definition of SELF-BELT
: a belt made of the same material as the garment with which it is worn"

http://www.zappos.com/esprit-denim-chambray-dress
"■Self string belt"

1) Yes, 共ベルト･ベルト通し付 would be “matching belt with belt loops" becomes it comes with a matching belt, etc...

2) A “共ひも” is usually a waist tie of a matching (using the same) fabric sewn up as a thin rope or ribbon-like tie, but sometimes it can be a thin cord or rope which is not necessarily of the same fabric – however, it “matches” or "comes along with" the outfit.

3) It's impossible to know for sure without actually viewing the item.

4) It is a loose thin tie for around the waist as contrasted to the conventional image of a "belt" which has a stiffer quality and often made with an inner support.

5) The last website that I’ve posted below is an example of a “matching waist tie” that does not use the same fabric.

6) I'm sure that there is a photo of the item in the "actuals" in that case, I think "waist tie" or "matching waist tie" would be sufficient.

Here is are some websites with outfits using various waist ties. They can be called waist tie belt or waist tie.
http://www.overstock.com/Clothing-Shoes/Tammy-Mars-Womens-Ti...
Tammy Mars Women's Tie Waist Striped Slub Tunic
Charcoal and black stripes accent this women's slub tee from Tammy Mars. This tunic features a flattering boat neck and a matching fabric tie at the waist.
http://www.thefind.com/apparel/info-tie-waist-tunic
Jewel tie-waist tunic

http://www.ardenb.com/catalog/product.jsp?categoryId=133&sub...
Tie Waist Tunic Tube

http://www.zappos.com/pure-simple-sleeveless-v-neck-dress-w-...
PURE & SIMPLE SLEEVELESS V-NECK DRESS W/ WAIST TIE


http://www.etsy.com/listing/48664432/light-sky-blue-white-pa...
Light Sky Blue White Paisley Print Tunic Dress - With Matching Waist Tie Belt
Comes with a thin white waist tie!