I stain the boards before putting them up. That way all areas (including against your horizontal rail) are protected. It's also easier IF you stain using a brush rather than a sprayer.
This is "green" (high moisture content) untreated fencing lumber.
Approximately 140 if using 1x6x6...
My nards carries them
Menards carries them
Using 15 feet=180 inches, I come up to 35 boards. This will allow you a couple boards for scrap. Always a shame to be one or two short and have to return to store to finish the project.
No, that is the fir boards
No. They are untreated and very green fence boards.You must let them dry. They will shrink 1/4 to 1/2". The photos shows boards that were butted tight together when nailed up.
No. They are green, untreated lumber. However, cedar, by nature, is pest and somewha rot resisant.