If that was true, the only acceptable translation would be "It is from between us", which would be incorrect. So Duo apparently has to accept an idiom for an idiom, or even a periphrastic expresion where there is no corresponing idiom in use, which then mutiplies the number of possibly acceptable answers, so some are missing. It's as exasperating as it is inavoidable, but that is a life. :)
I find the way the speaker emphasizes words to be confusing, like in this example. Are these good examples of how someone might actually use inflection (emphasis) in German sentences, or are they just basically the words being "put together" in a sentence. It sometimes reminds me of how my GPS speaks to me, with every word pre-entered ahead of time and then added together to make a sentence.
That's not what inflection is. Inflection is when a word changes form to express different grammatical categories such as tense, mood, voice, aspect, person, number, gender and case. The way a word is "emphasized" is not inflection, that's intonation. "Der" changing into "den" to express accusative case is an example of inflection.
Why does over mean "finished" in this particular sentence instead of "over?"
Prepositions almost never have literal translations in other languages, especially when they're used as particles (as they are here).
Edit: I'm pretty sure this is actually an adjective and not a particle but whatever.