I've been using Izotope Radius for time stretching and while it does a great job (so do a lot of other software solutions out there) I wouldn't use it for more than a factor of ±10% the original speed. The more extreme the change to pitch or tempo the more artifacts will be introduced, especially so when slowing things down as the software has to interpolate data that isn't originally there.
The type of source material also plays a major role of how far up or down you can stretch/pitch but generally at anything more than 10% up or down you start getting issues.
For your case, with a factor of two you might want to consider reshooting/re-recording the audio if at all possible.
Right. I figured the DP was overcranking for an effect, but you never know. No, you can't really expect to use the sync audio from the camera when you slow it down to 24, 25 or 29.97 fps. YOuhave to do that with ADR.