I think you should add "That women is making her second sandwich." as a possible awnser.
to say someone is in the act of doing something is -고 있어요 so no, that's not a possible answer.
How does one know that "second" modifies sandwich, not woman (as in a second woman makes a sandwich)? The grammar accompanying the lesson indicates that numbers tend to follow their noun: "The general usage is Noun+Number+Counter+Particle in a sentence."
Not when it comes to things with 번째. So words like "first" and "second" (첯 번째, 두 번째, etc), which are adjectives, come before the noun they are modifying . Thus it can be understood that it is modifying sandwich and not woman. We are not dealing with counting things here (which follows the rule you stated); don't confuse the two concepts.
샌드위치 두 개 먹어요. (I eat two sandwiches.)
두 번째 샌드위치 먹어요. (I eat the second sandwich.)
That is noticeable after you see some of the exercises. But there should be a note about that on the lesson's "Tips and notes".