Has anybody been into and/or out of Poacher Lake? Is the stream at the north end of the lake navigable to Sunday Lake? Are there established campsites on the lake?

Once, though it was a couple of years ago.  There's one campsite at the north end of the portage to Inlet Bay, as well as a short portage along the way to Sunday Lake. 

On my first trip to Q 40 + years ago my wife and another couple base camped on Sunday.  We made a day trip to poacher.  The creek had several beaver dams and other places where the canoe dragged bottom.  The water fall at Sunday was easy to get around. I do not remember seeing any camp sites. The creek that runs in from the east is much colder than the lake. This creek was too shallow to paddle. There are some big cliffs on the West side and the hill on the South side looked steep. 

Agree with what CG said.  I camped at the south end of Poacher, one September.  The portage over from Basswood a pleasant, long hike in the woods.  Remember seeing otter on the way out to Sunday, which was more paddle than portage.  Have friends who ice fish for lakers on Poacher in the winter.