How to run single ethernet (no PoE) in trench through garden?
The tower up the hill might take direct strikes before you, but those strikes might make big surges on the ground, so long story short, always use surge protection (or better, all dielectric fiber).  

You don't necessarily have to meet the code requirements for electrical installations with your direct buried ethernet cable and I wouldn't worry too much about 6" of burial except under a gravel driveway.  I don't think that will survive long.  I don't think PVC will help much, I'd sleeve it in galvanized rigid metallic conduit across the driveway and hope for the best.  (I am assuming this is just for home networking etc. and isn't anything life and death.)