Green Items and them being dropped.

Green items are not a garunteed drop. So are are slightly more rare than collectors items. I have made five or six runs into Sorrow's Furnace and have seen more rares picked up than green items. Personally 750(price of the key) gold for a rare. anet screwed up on that but I like the green "uniques."

Is is possible to get "greens" in Miner's Chests ? i didnt try yet...

Nope, greens only drop from their respective bosses, and even then not always.

only one per boss...?
cauz i saw screens of some of them..didnt see a sword yet