No, they don't have to adopt 3rd party platforms if 1 person has been elected to a 4 year seat. They just have to wait it out.
Don't think so? Just look at the career of Jessie Ventura as Governor of Minnesota. What changed with his election? Did the parties adopt any of his policies? Did anything change in either party?
Nope. They waited him out. Nothing changed. He wasn't able to pass any significant legislation because he had no support in the MN House or Senate.
Again, if you want proper 3rd party representation in the US system, elect candidates at the state level. You'll be able to build enough to get to the Federal level. Then once you have enough candidates elected to Congress, you'lll be able to get enough funding for the oval office.
Thinking that voting in a 3rd Party President and the other parties will magically start converting is a fantasy.