Right, but if you continue reading in Jacob, verse 30, it reads:
"For if I will, saith the Lord of Hosts, raise up seed unto me, I will command my people; otherwise they shall hearken unto these things."
When it says "otherwise they shall hearken unto these things" it is referring to the commandment that you just referred to regarding having one wife. Notice the use of the word "otherwise", this signifies that there are conditions in which the Lord will command to have more than one wife. The condition being "to raise up seed" unto Him (during the time of the restoration). So, according to the Book of Mormon, the rule of thumb is to have only one wife, unless otherwise commanded by God.
Regarding Joseph being only called to translate the book of mormon, you quote:
"And I will give unto him a commandment that he shall do none other work, save the work which I shall command him."
This is true, but what is the work that the Lord commands of Him?
Continue reading in 2 Nephi 3:11:
He will be a "seer" like unto Moses, "not to the bringing forth my word only, saith the Lord, but to the convincing them of my word"
Verse 24:
"And there shall be one mighty among them, who shall do much good, both in word and in deed, being an instrument in the hand for God, with exceeding faith, to work mighty wonders, and do that which is great in the sight of God, unto the bringing to pass much restoration unto the house of Israel"
From this we understand that he is much more than just a translator. It mentions that he will be a "seer". What constitutes a seer? Read in Mosiah 8:16, Amon says "a seer is a revelator and a prophet also; and a gift which is greater can no man have, except he should possess the power of God." Verse 17: "a seer can know of things which are past, and also of things which are to come, and by them shall all things be revealed."
So, Joseph will be a prophet, seer, and "revelator", not simply a translator. Revelators obviously receive revelations. The D&C is the book that proves it.
I don't have reference to the Book of Commandments, that verse is not contained in the modern day D&C, and I would have to read it in full context before I could comment on it. But, for the sake of argument, you are referencing the Book of Commandments as if it is true, yet you claim it is not, which is it? The very fact that he received that revelation shows that he has other gifts besides translating. He is receiving direct revelation, is that not another gift? There must be more behind that verse.