Whereabouts on the Kohen Gadol's body was the Eifod worn?
Rashi and Seforno (on Pasuk 6): The Eifod was worn on the back of the Kohen Gadol, from the waist (corresponding to the elbows) down to the ground. 1
How were the two shoulder-straps attached to the Eifod?
Rashi: They were sewn on to it. 1
Rashbam: Besides being attached to the Cheishev ha'Eifod both at the back and in front, the shoulder-straps were also attached to each other, 2 so that between the Eifod, the straps and the Choshen, they covered the entire body of the Kohen Gadol.
One wonders why the straps were not woven together with the Eifod like the belt. Particularly since they were made of the same materials as it. Refer to 28:8:2:1.
Otherwise, in the course of the Avodah, the straps would likely have slipped off the Kohen Gadol's shoulders, and the Choshen would have fallen (Rashbam).
Having stated "Sh'tei Cheseifos Chovros", why does the Torah add "ve'Chubar"?
Oznayim la'Torah: This goes nicely with the Rashbam (Refer to 28:7:2:1) - that the straps were also joined to the front of the belt, and the Pasuk is teaching us that, each time the Kohen Gadol put on the Choshen, he had to fix it to the belt of the Eifod.