Products was not designed for such use, and there is no warranty for such application.
I wouldn't. It is not meant to be used outside.
Not advisable. You should use some flavor of exterior plywood -- and be sure to paint it with a suitable exterior paint.
I use Alex paintable caulking for everything and buy it by the case to save a little bit of money. This may not be prefered but I use my finger to smooth the caulk after it is applied. With practice you can apply the right amount of caulk so you get a smooth seam. You won't be able to sand or smooth the caulk after it dries so make sure it looks good before you let it set up. Wet paper towels can also be used to smooth the culking or to wipe off access. Large temperature swings could cause your boards to expand and contract. Tongue and groove would be the best option to allow movement during changing conditions. The caulk should provide elasticity but may dry out and crack over time requiring re-application. Sounds like a fun project!
I prefer not to paint it at all, and use it only in applications not requiring paint. The rough side is difficult to paint, and the smooth side is sufficiently smooth that paint is unlikely to adhere well. Home Depot sells a very nice thin plywood which would be suitable for any indoor application, and the price is reasonable. I used some to build up a subfloor of 1/2" plywood to match the surrounding subfloor of 5/8" plywood.
I would not recommend this product for any flooring: it is too easily scratched. I use laminate for interior flooring; it is coated with a thin layer of aluminum oxide (not visible) which is very hard and resists scratching. For exterior work, I like Veranda -- but it is not suitable for installation over an existing floor. For your case, I would simply paint the T&G with a suitable deck paint.
No Eucaboard, is not sound proof
I would not do this: the product isn't very attractive, and is too easily scratched. I suggest a nice plywood, which can be finished in a variety of ways.
This would work - sort of. There will need to be an adequate frame to support the panel, and with a 4 foot width that will be a problem. An easy dodge: paint the mirror (use an exterior grade flat finish paint), and then put the wallpaper on top of the paint. The paint will provide a suitable surface for wallpaper glue to adhere.
It depends on of kind of finish you may want to achieve. Something smooth close to the industrial finish, yes sand with 120 gr or higher, prime and pre-finish. If you are Ok with small imperfections then just prime and paint – If all you need is a temporary panel then just pre-finished without primer. Remember to use good quality paint latex acrylic or alkyd melamine.