Re: changing course, as long as your generation ship accelerates for more than one human lifespan, your future generations of involuntary colonists aren't going to have a lot of options besides going along with it. The choice wouldn't so much be "keep going or veer off and colonize a closer star system" as it would be "keep going for another 800 generations or veer off and colonize a closer star system in another 80 generations". I mean, there's probably no way to stop them from making that choice, but it's hard to see anyone caring whether their nth-great-grandchildren die with one or ten millennia left on the trip.
If the ship accelerates for the first 100 generations, the 2nd generation likely still has every chance to target a different planet. I don't know which generation would be the last, but I'd need to see actual math to believe that it's strictly only the first generation.