Bus up the expressway from Nanchang to Lushan (more frequent departures if you change in Jiujiang which is nearest sizeable city/transport hub). You're the one who earlier mentioned you wanted to focus on natural areas not cities, it is now ???? why you suddenly want to divert and spend time in Nanchang--except that your less-than-casual friend will serve as a free guide. Well, OK if that's what you want. What I'm trying to say is that if you stay with your Nature over Cities theme and make a brief stop in Nanchang then go to Lushan for a day or so (with your friend if his/her schedule permits), it makes more sense to not backtrack to Nanchang but to continue on to Wuhan, where even cheaper flights to Beijing are on offer.

I think on this thread, we've just about reached the limit of being able to help you since we can't make value judgments on destinations for you. All we can tell you is when your logistics aren't realistic.