I'm pretty sure that you probably have all of the legal issues settled already.
As far as how to broach the subject of end of life care decisions, that is specific to every individual. You stated that every time you bring it up, they deflect. There is probably a reason for that. Either they don't want to face the issue, or every time they talk about it, they get into a disagreement as to what each wants to do. Try talking one on one to each of them separately and see if they open up. If they don't or won't, just state your opinion on the matter. If you place it out there that you will feel better if they were closer, what can it hurt. You have expressed your opinion on the matter in a non-confrontational way and it is on the table. In the end it is their decision for now. If they become incompetent or unable to care for themselves in the near future, then I feel you should be more aggressive in getting your point across. Once they can't make the decisions, then the ball is in your court.
These things usually have a way of working themselves out.