I don't quite remember learning לתוך. This "mark all correct translations" is not a very good feature.
Sometimes we put some non-learned words as a possible best answer by mistake. So they appear in that exercise. When we come across them, we remove them. Anyways, thanks for reporting!
Thanks! It's not that big of an issue, but frustrating sometimes (like when you're already doing bad because you can't remember things you are trying to review).
Should "הוא" be "זה" instead?
I have the same question. FWIW, הוא here would be correct if referring to the key ("the key isn't working"), rather than the action of trying. But that seems like an unnatural interpretation of the sentence.