A well defined doc evolves over time. it gets sharper with real-world scenarios, incidents, and experiments. Before Voyager 1, we didn’t have that kind of experience. You can’t predict everything upfront.
Not really. Jupiter alone is good enough. Its huge mass accounts for almost all of the gain you get from any such slingshot. Launch windows from Jupiter to anywhere occur every 12 years. Voyager's alignment was captivating, but realistically if it hadn't happened, we would have just done separate Jupiter-Uranus and Jupiter-Neptune missions instead.