I used the 39 SF number as you should allow for drop and unusable pieces. If you try to order exactly what you need you will undoubtedly be short of material.
Use a finish nailer with 1 1/4" 18 ga nails. Nail thru the tongue; try not to nail on the surface as these will be noticeable when you look at the finished job. Ends need to come in the middle of the ceiling joists or trusses (as the case may be). Be sure to stagger the ends and not have two boards in a row ending on the same ceiling joist. For your first course, pop a chalk line to make sure your first row is straight. The grove goes against the wall; the tongue goes on the chalk line. Subsequent course have the grove go into the tongue of the previous course. Make sure each piece is tight against the end of the piece you just put in. Also make sure to push the grove tight against the tongue. Nail from one end of the piece you're putting in to the other end, one finish nail per each ceiling joist. If your piece of material is slightly warped, you might have to skip a few joists in order to push and nail further down the board to work the warp out of it. Do NOT try to put this up with a hammer and finish nails by hand. You will damage the boards plus it will take forever to get it done. If you don't have a compressor and a finish nailer, go buy or rent one. I personally like Paslode but, honestly, any of the reputable air gun makers will do. I managed to get a pancake compressor with a pair of finish nail guns, on sale at Home Depot, for a really good price. Look for sales if you're going to buy. You can usually get a pretty good deal. Whether you buy or rent, don't forget to buy a couple of packs of finish nails that work with your nail guy. 18 ga x 1 1/4" finish nails are a good size for this product.
No, the ends are smooth.
I strongly recommend installing over drywall for rigidity and for code compliance. Most areas require 5/8" fire code drywall in ceilings.
Yes, if you apply coats of spar varnish after filling the knot holes.
This paneling is on 8 foot lengths only- if you need something longer and you don't want any butt joints, go with clear cedar which comes in long lengths.
I installed this in two separate rooms, but on the walls- not the ceiling. I'd be worried about a nail popping out as the weather changes, which could lead to multiple boards pulling away from the surface. I would recommend using a construction adhesive with or without nails. I used 6 penny finishing nails when there was a stud behind the paneling even though I'm confident that the construction adhesive alone would be fine.
It's not clear pine, and there are a lot of knots. Most of the knots are intact; I had to get creative with putty and stain to fill in the missing knots. But with the knots comes a beautiful grain that really "pops" when you darken it a couple of shades and give it a good coat of varnish.