TL;DR: No computer is a good translator. Duolingo is more likely to be right than Google Translate. Also I expand on mukkapazza explanation of Forse no.
Google translate is okay - for a computer. I don't think there is a better one available on the web. But no computer is a good translator. The only good translators are people.
A computer, no matter how well programmed, can't have anywhere near the understanding of the language a human can. The computer just takes the most likely possibility for what you typed in and gives you that. It can't understand nuance, natural usage, idiomatic phrases, phrases where there are two possible meanings, phrases where a literal translation wouldn't be correct, etc
That's why using Google Translate to question Duolingo is probably not a good idea - Duolingo is more likely to be right.
It might not always be right, but the fact that humans have thought about each sentence-answer pair, and each sentence-answer pair has been checked by multiple other humans doing the course means that it's likely to be right. The humans using Duolingo have differing levels of ability of course, but it's likely that the only people who pick up on a mistakes are ones who already have a decent background in the language. For myself (using Duolingo to re-build all the vocab I've forgotten since studying Italian at uni over 7 years ago, but still have a fairly good understanding of grammar) if I pick on something I think is wrong - I cross check to Italian grammar books/sites before reporting it.
I've only noticed one mistake so far and that was in the English translation of a phrase.
It's good to ask questions when you're learning - it helps you learn. So go ahead :) But I recommend trusting Duolingo over Google Translate.
Regarding Forse no, as mukkapazza has said - this is a usage issue: Forse no - can be used in Italian as a sentence on its own. Forse non - can't be used as a sentence on its own.
In English "maybe not." can be used as a sentence on its own. "Maybe no" - isn't something you really say in English (certainly not as often as Maybe not.).
Therefore Maybe not = Forse no (even though they aren't literally equivalent - they are still equivalent because in a situation where an English speaker would say "Maybe not." an Italian speaker would say "Forse no.")
Hope that helps :)
Automated translators have no concept of context. That is probably their biggest failing, and one that cannot be corrected. However, they can be useful, as I have learned most of my pre-duolingo Italian using the Bing translator. Of course, I was aware of its faults, and whenever I used it, I double-checked everything (not really possible unless you have something to get it against). I have 3 books I use - a basic grammar book, a Cassell's Italian-English dictionary, and a book of idioms. When using a translator, I always translate the Italian back into English, which sometimes points out some errors, but often makes a lot of whopping big errors (it's far worse than the translating into Italian). And not just Italian, I tried it a couple of times with German, and I am far more familiar with that, and the English translations are awful.
While Google will resort to word for word if it needs to, it does not always do that. Try asking it to translate 'I played an April fool's joke on him' and you will see that Google clearly understands some phrases. You are correct certainly about the word for word translation is a poor choice for confirming some fact when studying with duolingo, as opposed to using a grammar book or dictionary. But let's not forget that google translate is absolutely awesome when you need to translate something fast, say some paragraph, from language X to language Y, juat to get the flavor of the paragraph. Then Google Translate is handy to have. :-)
By the way, you are also correct about the difficulty of translation. Translation (and language understanding in general) is considered to be one of the things to conquer in artificial intelligence.