Is 0.0036% time inaccuracy on a consumer-grade camcorder expected?
IME, 0.0036% is rather better than average. Consumer gear operates on quite inexpensive crystal oscillators. These oscillators or crystals are not tuned precisely (or they would cost too much), and they don't have extraordinary stability over temperature ranges.  

Professional gear uses special techniques to keep audio and video gear in synchronization.  They use premium-quality sync generators with oscillators that cost more than your whole camera. And even then, they must be re-synchronized several times per day.

And traditional broadcast gear would have a master sync generator, and all the cameras, switcher, etc. would "slave" to that central reference so that everything would be in lock-step.

It sounds like your gear is operating properly. Nothing is "broken". That is simply normal and expected performance.

If you need to sync video to your reference audio track, it is MUCH better to "stretch" or "shrink" the video to fit the audio than vice-versa. Slightly changing the length of the video track with simply insert (or remove) an extra frame every few minutes, which almost nobody will notice.  But slightly changing the length of an audio track is likely to create quite noticable and annoying artifacts.  

It is simple enough to try it for yourself. Try slightly changing the length of a long video clip and watch the results.  And then try the same with an audio track. You may be lucky that you can get away with it. Only you can try the experiment with YOUR content and YOUR editing system, and YOUR expectations.