Consider adding a pop-off relief valve at the pump in case something gets clogged or kinked. This way the pressure is released and your pump does not deadhead which could cause it to overheat.
Ball valves will work, but actual flow control valves will allow a finer tune, if needed, be more consistent/repeatable and not so touchy should you want to balance between the diffusers.
I am wondering why you are putting in check valves since your pump is up hill from the pond and no real chance of siphoning water to the pump? Does it have something to do with a piston pump? My system has a rotary vane pump.
Your sizing looks good, you might even be able to add a third diffuser. A pump curve would help determine this. Can you post the curve?
You do not have to bury the line for any reason that I can think of IF you can establish a constant down hill slope to the pond so that any condensation runs on out. Unless you might hit it with the lawn mower
I would want to get it below the frost line at the pond so that pond water (when not in use) does not freeze in the lines and burst them.
EDIT: I'm not sure about the cost difference, but a lot of folks use the flexible underground line instead of PVC pipe. My pump house was about 40 feet from the pond so I put my valves in the pump house and ran separate pex lines into the water (underground) where I adapted to the sinking hose.
