Suggestions for the future
As you can see from my profile, I am a Duolingo enthusiast.
As I have worked my way through, I have realised that the multiple choice questions often don't really do anything for learning - very often the options simply test whether you can pick out the only sentence with correct English grammar, or the only sentence that means anything sensible in English. This does nothing for my proficiency in the languages I am learning.
This seems to me like an opportunity to use some of these wasted slots to cover word order, which currently gets no direct attention - perhaps starting from the point of introduction of negatives and adverbs.
Another good use of these questions would be to teach which prepositions follow which verbs - another neglected area, but an important one in many languages.
And the final point on my German wishlist would be practice in separable verbs, as a separate skill or through the multiple choice questions.
One thing I would like to see introduced is a review feature at the end of a timed practise. It's stupid looking at mistakes we've made while the clock is ticking, so we skip past and onto the next question. It would make sense if we could look at our incorrect answers afterwards!
Another thing that would help me a lot would be to see all correct answers of a translation. I mean the sentences you get shown when you get it wrong, I would like to see them also when I get it right, as "other correct answers". These examples give me a lot of new information about possible word order, spelling or expressing something in other words than what I had known. They give me a more complete picture of the language and sometimes show different angles of seeing things or additional meanings of words. And it would be good to have them in both languages as I am no native speaker and far from being perfect in English - so I am learning Spanish and improving my English here on Duolingo at the same time.
I think the multiple choices are fine when you're being asked to translate INTO the language you're trying to learn. Picking out, as you say, the only one that means anything or has sensible grammar is something you have to learn in that direction. I agree though that multiple choice questions that that ask you to choose among options in your native language are pointless.
My wishlist for Spanish would be more adverbs, more time spent on the imperfect, and considerably more practice with passive and reflexive sentences.
Also, basically any additional vocabulary section set between or after all the verb tense/mood sections that are bunched together at the end. The conditional perfect for instance seems fairly rare and doesn't justify much more focus than the little it gets, but there need to be some lessons where it can come up occasionally without being the focus so you can learn to recognize it without being told that's what you're practicing. The same basic idea goes for all the other tenses/moods/etc you learn at the end of the current tree.