Re: Roof Joists and Uneven Rafters
As far as removing the joists, it will depend on how the rafters were installed. The joists may provide lateral integrity for the wall. In other words they may be what prevents the joists and roof from pushing the wall out. Some modifications may be necessary.
And as Jkirk pointed out, installing shimmed furring strips, or strapping as you called them, would be the way to achieve a level platform for new drywall.
Be sure you live your life, because you are a long time dead.-Scottish Proverb