Does the Extraterrestrial Alliance always get the candidate elected it favors, or does this really make little difference as all elected officials can be corrupted or manipulated to do their bidding?

This does not always happen, as you know yourself in understanding the history of the John F. Kennedy Presidency, but the fact he ended up being assassinated was simply another way to sideline him when he proved too difficult to manage. So they are not perfect in either producing an orchestrated effect or using any particular strategy. It all depends on probabilities, the level of vulnerability and weakness in those being manipulated, and other energies, including a counterbalance effort by the divine realm to right the wrongs, to prevent a difficult situation from worsening, and so on. This is largely a consequence of prayers being mounted to help the world improve, or to cause a particular outcome in a difficult time or place that gets people involved in wanting to help. There is no overestimating the power and benefit of human intervention in response to inner spiritual awareness of what is best and wanting the Divine to assist—it is the most powerful tool you have and can make anything happen if enough of you come together in a common quest. The problem at the moment is too few are asking and there is still much confusion because of the high level of manipulation in place. So people are very fragmented and not pulling together because they have differing views and visions of the future needed. This is the wisdom of the divide and conquer strategy that has been ongoing for millennia, and keeps humans weakened and vulnerable as a result.