Yes you can, make sure you put the inner cover on top of the brood box first though.
You don’t have to treat western red cedar with anything at all. A lot of hives in the UK are made from it, and it weathers naturally to a nice silver gray colour. It will not rot, and most insects will not eat it. If you do treat it, only treat the outside of the hive - the bees will treat the inside themselves. Also, if you seal it, make sure you use something non-toxic or food safe - the sealant may come into contact with food (honey).
The location looks wonderful. Nice hive stand! I don’t think it is too close to the fence from the bees’ perspective - not sure about what the cows think, though!