No.13109827 ViewReplyOriginalReport
Lelouch could have saved Shirley using the Geass, he just did it wrong. The Geass cannot make someone do something that's physically impossible. Thus the order "Don't die" is void because there is nothing she could have done to help herself, plus any human must die eventually.
However, the Geass could order someone to accomplish some possible task the performance of which incidentally accomplishes an otherwise impossible result.

Suppose Lelouch ordered Shirley to "Sing the Brittanian national anthem 1000 times." This is a valid order, she would be forced to perform it and incidentally live long enough to receive medical help.

Light would have figured that out, Lelouch is a poor derivative.