Juts negril will do a one way for 18, we've done it many times and never had to wait long. As far as August, you can get some heavy duty rain at times, but like all other days in negril, it comes and goes.
Good tip about the one way tickets. We just moved to the bay area and had shell shock When looking at fares. I'll be anxious to hear how that works out.
And lastly, congratulations. A bad day in negril is better than a good day at work!