I have a series (and hugh) question.
If I am on a plane, getting ready to pull out, and someone boards that makes me feel uncomfortable, would I be able to leave the plane and transfer to another flight? I mean, without losing my money and having to pay for another ticket?
I believe that what you are suggesting is not possible, as it would instantly undermine the procedures and 'order' that the airlines have in place.
If, while the plane was loading or taxiing you were to absolutely throw a screaming, shouting fit about the matter, you would be regarded as someone 'interfering with flight operations' and it would be considered a criminal matter. The plane would then return to the terminal, ALL passengers would be deplaned and rescreened, ALL baggage would be unloaded and thoroughly checked. You would have several hundred people absolutely furious with you because you have just caused them to miss their connecting flights and pickups at their destination, work responsibilities as well as their entire vacations in many cases, but that would be the least of your worries because you would be VERY roughly arrested and subjected to the maximum penalties possible. They would definitely want to make a very lasting and very public example of you.
This sort of question is best to work through in your mind prior to buying your ticket. If you don't want to fly with 'anyone' in particular, then it's best that you make use of the splendid American Superhighway System, where you have true freedom of association and you can provide for your own security.