Your best bet is to find a "Type Club" for the Miniplane online. Whatever questions you have will be best answered by people who know the type.
As to the fin offset; yes you need it to counteract the torque/spiraling slipstream supplied by the motor/prop. It will keep you from having to bias the rudder to net the same effect if you install the fin with 0* offset. Don't worry about the rig changing when you're inverted, if the motor is turning the same way the fin offset will be the same...
The Smith is a short coupled airframe and will therefore have a more acute offset angle than an airframe with a longer distance from the CG to the surface. All prop driven airplanes have some way of dealing with torque; offset, "airfoiled" vertical fins (low pressure on one side) or a combination of the two.
I once heard of a fellow who decided that zeroing all the incidents out of a Lazer 200 would make it fly the same up, down, left and right and built it that way because it was "logical". The airplane was basically uncontrollable scrap from day one.
My Hiperbipe is short coupled, has a fuzz over 200 horsepressure and has (I think) 4* fin offset.
Hope this helps ya'