To stay down just push no buttons to just lay there, to roll back I believe tap either 1 or 2, I think both work, and each character can wake up with special moves. . . of course some being better then others depending on the situation.
Hold down to stay on the ground for a short time. You can also press a single button, such as Front punch of front kick and so on, when you are about to stand up and your character will roll backwards. And of course you can also do a wake up attack by inputting the command when you are about to stand up.
Yes, Mileena's back 2 is unblockable when laying as it hits overhead and opponents on the ground, not sure of any others. You can't use wakeup attacks after staying down.
Yes, Mileena's back 2 is unblockable when laying as it hits overhead and opponents on the ground, not sure of any others. You can't use wakeup attacks after staying down.