I am not sure about your other questions, but I know I would use 500 milers over miles. If you can't be confirmed now, then it is highly likely that you'll go onto the DM list with all the other upgraders. At this point, miles of 500's make no difference (although status does).

To me at least, the 15k (or 8k if you are Y/B) is worth much more than 5 500's. Then again, I do mostly international or domestic connections from international, and don't really get to use my 500's that much.