Rush won’t render audio track

Like other Rush posters I can see in terms of urgency, my project was due yesterday but I'm stuck on malfunctioning rendering. I've literally spent hours, each time emptying cache, quitting Rush, then restarting holding alt key (Windows) to clear preferences. It's still hit and miss, especially as I want to continue making minor tweaks. I have a complex video (for Rush) with 4 video tracks and three audio. I changed audio by shifting clips around so I used only two tracks. But the problem persists. On export, one of my audio tracks is missing. This is a disaster! If I go through the steps of clearing the cache and preferences, it's then hit or miss whether both tracks will export. The audio consists of 1 music track composed of two long clips (was two tracks, as originally I had some overlapping, but I amalgamated them into one, in case Rush was not coping) and 1 track composed of multiple consecutive recorded speech clips. I've seen this issue around the internet and with a render time of 15+ minutes for a 4.5 minute clip, Rush is failing me big time. I'm writing this while waiting for another export to complete and don't know whether both my tracks will export or not. On top of this - and I don't know whether this is a cache issue - my audio plays back in the editor at a different volume each time I return to the video after clearing Rush. I'm trying to achieve a custom balance between my audio and video; I want one of the music clips to start much louder than the previous one, so can't rely on auto settings - and anyway, why should I? After testing my latest export my next move is to import into Premiere Pro and export from there but I'm worried it won't transfer accurately.