A: Pure stain should be either applied with a brush and wiped off or just applied with a rag. Rolling will apply too much unless you plan to wipe off the excess. However, if you are using a stain mixed with Urethane, that can be rolled on.
A: Unfortunately, no.
A: Yes, but before you use one, you should slap it against your free hand a few times. Initially, you will see a few lint particles fly off the pad. After a few slaps, you won't see any more lint, and you can use it lint free.
A: It is a terry cloth wrapped sponge, both washable. Obviously, whether it comes sufficiently clean to reuse will be determined by what it was used for. Oil-based stains and paints being more permanent than easier to clean products.
A: They could. I would think the average deck is too large to do it with these size pads by hand, but...