I do think having it translated as "One liter is not too much" is more correct in this situation, because firstly we've recently learned the word "unu" (so it's more practice for that), and secondly because it's a more literal translation for the sentence. Although both sentence variations do mean the same thing fundamentally.
While "a lot" is technically a noun, note that it is often used as an adverb as well in English.
- It's a lot harder than it looks.
(Here "a lot" clearly modifies the adjective "harder")
- I go swimming a lot.
(Here "a lot" is used to mean "often" or "frequently". You don't swim a multitude of things.)
But yeah. In this particular case it does not make a lot of sense from an English point of view.
Hmmm, I'm still not sure I see it, because to me in "dos litros no es mucho", "mucho" in that case is still a noun, like another way to say the same thing would be "dos litros no es una cantidad muy grande"...it stays the same as a singular noun. Unless my Spanish is wrong haha, which is possible...
No. Unfortunately, I'm not able to explain why. :/ I believe it has to do with countable vs uncountable nouns, as per the explanation given here: http://www.differencebetween.net/language/difference-between-much-and-many/
No. If it had been something like, for example "One litre is not a large quantity", then yes it would be "granda kvanto". But here, it really means "One litre is not much (of whatever liquid we are talking about)", so it is "multe".