Yes, but I would expect to only last several years u like treated wood but treated wood uses toxins you should not, have near food. Personally, I would use more then one on each side to get a proper depth of soil. You should also stain or paint them to extend how long the will last since it will resist moisture much better. Especially treat the ends, that is where water most easily enters any wood. This happen with siding all the time because the lazy contractors do not paint the end before installing the wood. They just spray afterward which only protects the sides. Water then runs down and flows past the ends and enters the wood and rots it from the inside. All my neighbors have rotten Douglas Fir siding for exactly this reason. My cedar siding is just fine.
This wood has not been treated, that is the advantage of cedar. It only need staining or priming and painting of used for an exterior application. It is a very rough surface so I would sand it for you application.
100 lineal feet times 12 inches in a foot equal 1200 inches divided by 5.5 the width of a fence board equal 219 boards for full coverage no spacing. The boards after several months will shrink and naturally space. check the math!!!!!!!
I do not believe Home Depot carries this wood in that length. You have to go for the much more expensive and thicker cedar to get those lengths. Dimension cedar is real nice wood but not cheap.
Not really. I would stain the ends immediately, install and then stain them. They will resist damage longer if stained. Do as light a coat as you can. You can always darken it later of you want.
NO, that is WHY you use cedar instead of treated wood. Good for garden boxes and applications around animals.