Which then leads to an interesting question: why is "de" needed?
I think it's the same reason why we don't say "much our food" instead of "much of our food".
In this case, it limits the amount of the specific noun. "Mucha" is acting like a noun that is modified by "nuestra comida", which is also a noun and explains why the preposition is need. If you exclude the quantifier, it would be like saying all of our food. Here are some examples that follow the same pattern:
"Algunos de nuestros hijos están en la escuela" -> "Some of our children are at school" (not all of them)
"Tres de los hombres se levantan" -> "Three of the men are getting up"
"dos de nuestras bebidas son deliciosas" -> "two of our drinks are delicious"
I think that certain "adjectives" that undergo apocopation do not do so in these situations because it isn't immediately before a noun.
- "Uno of mis boligrafos está en el baño" -> "one of my pens is in the bathroom" (despite it refering to a masculine noun, it doesn't undergo apocopation)
"Mucho" can be hard to translate into English. "They enjoyed much our food" doesn't work, and "They enjoyed our food much" isn't any better. "They enjoyed our food a lot" is OK but attracts criticism from some who maintain that lot means a piece of land. (It doesn't, it means a portion.) Duo likes the word really for much. It is a useful word. Of course literally it means in actual fact, but colloquially it means "a lot" or "very much". It is IMO the best solution.
Your translation may have been rejected if you included 'very' since 'muy' is not in their sentence. But in my part of the world, it would not be unusual at all to say 'they much enjoyed our food', but that was rejected as well. I reported it on 7-7-20 for them to consider adding to their database.
Yeah, sure, without context, the imperfect could be used here as well. The Spanish sentence seems to refer to just a single instance here.
Please note that the imperfect is not a different mood, just a different aspect.
- tense - relation between time of speech and time of action (past, present, future)
- aspect - temporal structure within the action (perfective, imperfective, progressive)
- mood - communicating the speaker's intentions or beliefs (indicative (statements), imperative (orders), subjunctive (hypotheticals), conditional)