The triggered ability looks at whether you've gained life in the turn, even if Witch of the Moors wasn't on the battlefield when you gained life. It doesn't care if you also lost life, even if you lost more life than you gained.

You don't have to choose a target creature card at all. If you do and the target card is an illegal target by the time the ability tries to resolve, the ability won't resolve. Your opponents won't sacrifice creatures.

The triggered ability triggers just once, no matter how much life you've gained.

As the triggered ability resolves, first the next opponent in turn order chooses a creature they control, then each other opponent in turn order does the same knowing the choices made before them. Then all the chosen creatures are sacrificed at the same time.

If you haven't gained life by the time your end step begins, the triggered ability won't trigger at all.