I don't think you can use the example of a sphere for the orbit. That only becomes relevant if the craft is going to leave the plane of the ecliptic and surely that would a Bad Thing(tm). Space is three-dimensional but pretty much everything within a solar system is on the same plane. Presumably it requires more fuel to leave the PoE than staying within it and since there's bugger all there for anyone to visit you'd be on your own. It would be like a trip from London to New York via the Moon. That might avoid issues crossing the Atlantic but if you breakdown you're a long way from help :)
As for the general idea of 'easy to miss a planet' doesn't that depend on your schedule? You might not have fuel constraints with a sail but you will presumably have time constraints. You might not have any practical choice. Sling shotting would still be a useful technique.
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