It's a tough situation because of limited height in the attic, the distance to the lumber, and the age of the lumber. Maybe you could try an auger bit which will cut better...with the set-screw type heavy-shank extensions which won't break. You could pre-assemble extensions if there is enough height in the attic. This way there is more time in setup, but once you get there the hole drilling will be easy...with a right-angle drill.
Although I've never done it, you're job might be a little easier if you gave yourself a generous slot in the upper plate. You could take out quite a bit of the plate (1" X 12") which would allow more angle in feeding the extensions, as well as a better view.
Dave
