Why are some of the hints wrong? Why is it when you use the hints given, the answer is wrong. This is very frustrating.
The first meaning in the list doesn't necessarily have to be the right one for the current sentence or you might have to change a word to capture its meaning. You are right that it is sometimes impossible to guess the meaning of an idiom as its structure and vocabulary can be very different from the other language. Most of the time you should be able to get by if you think about what would make a natural sentence. Just don't expect to get a word for word perfect translation from the computer whenever you have to peek. Duolingo was created exactly because computers aren't perfect at putting words together.
I understand how I might be led astray by a peek. However, it seems as though if I go to the vocabulary list, I should be able to get enough information to understand what the sentence means. Right now, I'm going to an outside website to find a more complete definition for a word I don't understand in a particular context. Furthermore, I don't feel confident that the sentence is really idiomatically correct in Spanish. This has been rather frustrating.
A good dictionary is always a separate thing; a site or a book. Vocabularies given on language learning sites or in textbooks are pretty limited. The more you learn, the more you should use a good big dictionary. While Duolingo aims at being the best site for language learning, it would be really a waste of time and efforts for Duolingo to develop a big comprehensive dictionary when there are a lot of them outside. Vocabulary list is not perfect, I agree, and it will be improved, but certainly not up to the level of a real standalone dictionary.
As for your concerns about idiomatic correctness, discussions are really helpful for clarifying all your doubts as there are enough native and fluent Spanish speakers here.