For the first question:
It's a funny thing really. From everything I've read Joseph used a number of means to translate the Book of Mormon, so it's quite possible that all the paintings are accurate. Then again, maybe the artists simply wanted to show him translating w/o any real concern for the accuracy. Unfortunately for us, we really can't know just how accurate they were since we have very little info to go off of.
For the second question:
Some sign their pictures, others don't. I've seen both, and many times, when you see reproductions of LDS pictures, what you're seeing is only a part of the original. So it's quite possible that it has been signed, but the signed portion doesn't appear in the reproduction.
As for the third:
I haven't a clue. I know of no such rules, but then I wasn't around then so I have no idea. It may have been possible for the man to be with all of his wives at once, but I haven't any clue whether it was considered proper or not. My GUESS, is that it was a one-on-one thing between that specific woman and her husband, as in many cases the wives all had different houses.