Properly Venting a Dryer
Just that a long vertical can result in more cold air injection--the weight of a column creates more downforce. The same is true in reverse for hot-attic days. Also, as you can plainly see, dryers emit lint. You might consider a soffit vent to keep the lint off your roof. 
Nope. Pack it snug to the vent pipe.
Nope. Fire is not a concern here.