To solve the first problem, I decided to print the shingles on my home inkjet printer. Then I sprayed the whole sheet with fixative so I wouldn't have to worry if the water-based glue smeared a little.
I solved the second problem by opening the GIMP and adding some horizontal lines to the underlayer of the roof. It was a little like snapping a chalked line would be in the real world.
I let just a bit of the shingles hang off the edge, then trimmed them back to the thickness of the green trim I'm using on the rest of the building. When I add the trim, it will go just under the shingles.
I edged all the cardstock with a neutral grey Prismacolor felt tip pen. Bleeds a little, but not enough to be seen on the printed side.
I cut off about 900 little white triangles of paper in the making of the shingle strips.