So your bow season starts late huh....we opened two weeks ago, guess we have a really bad deer issue.
The piping you are increasing is usually not an issue if it is only one size bigger than the opening at the stove, just do not decrease in size.
I have had wood stoves all my life with only two chimney fires, and I cleaned the flues every two months. Once I switched over to the double walled insulated pipe(which is expensive) I do it once a year.
Masonary chimneys are really not made for wood stove use. They take to long to heat up and if they are on an exterior wall build up creosote much faster than an interior chimney.
As far as being safe with the stove, building a wall around the area is fine, you creat a large thermal mass that continues to hold heat after the stove is done burning. A better thing to have would be a carbon monoxide and smoke detector in the living areas, the house can always be rebuilt.
Another thing to think about is the weight of the block around the stove, unless of course it is on a concrete floor.
Good luck , check out www.woodheat.org
they have a good site.