Dont know on the second one. WOuld have to watch it again and that would be waiting till dvd.

As for the first question is because she became the Soul stone. Since she was the stone. bringing her back would be impossible. Hence Grimora is still alive and not Natasha. Because in the 'past' Thanos didnt get to kill Grimora to get the stone. Clint and Natasha beat him to it. And when Bruce used it at first, he admitted that he tried but didnt succeed.