Re: framing issues
It's not necessarily a poor design, if the joists were sturdy enough; in your case, they were not. Even as I was reading the description, I was waiting for you to say that there was a vertical member in line with the wall plate supporting each rafter ; I agree that adding one now can only forestall worsening of the situation. There is no force you could apply that would be able to straighten out the bows that have been seasoned into your framing for many decades. You could sister bowed members (after pulling all surfaces) so that straight lines could be achieved. Conversely, things that bowed down could be planed straight, after the skin was removed, and the necessary row of pony studs got installed.
Remove not the ancient landmark, which your fathers have set.