For the answer to the questions above,
Schindler's promise to them is that they won't be put to some concentration camps or transported here and there until they grow exhausted and die. Instead, he hires them to his factory where they are making ammunition for the war except they are just a front show for the Germans.
Because they will be safer and saved from a lot of suffering. Nazi's plan is to eliminate and make the Jews work until they suffer and die. So, instead of that fate, the Jews chose to invest in the plans of Schindler and many Jews are survived from the Holocaust in return