The most accurate one is the wordreference one, and for human voices the one on the example in forvo for "Piacere di incontrarti." by betpao. At least according to my dialect/accent, which keeps the first "e" closed. I'm not sure about linguistics, but if the "e" were to be open, the best examples would be for "piacere" by beahpb (which sounds actually a bit mixed) and for "Il piacere è tutto mio." by giorgiospizzi (completely open "e") on forvo.
In any case the duolingo one sounds a bit artificial and weird, you're right.
In italian to ask for a favour one can say either "Mi faresti un piacere (+", cortesemente" to be even more polite)? or "Mi fai un piacere?" to be less polite but still correct. So either conditional, for optimal politeness, or indicative for neutral politeness. With the imperative "Fammi un piacere!" you can only order people to do stuff, so it's reasonable to say it's impolite.
Moreover, usage of imperative and a different intonation on this sentence results in "Ma fammi un/il piacere..!" (= "Ma per favore..!" = "Ma cosa vai dicendo?!") which implies the person you're talking with is talking bullsh*t, saying stuff that isn't true, etc and is often accompanied but that wonderful hand gesture some italians do with all the fingertips united and an up-and-down movement. In that sense, usage of imperative is highly discouraged if you're actually asking for a favour.
Ok, that makes sense: with the third person + imperative, there are some situational conditions in which it's accepted as neutral. First one that comes to mind is when an elderly person asks for something to younger people, officers of some sort, other...but I'm pretty sure anybody could use it.
Still, it already implies that the other person must accept, so you're not asking if he/she can, you're kind of forcing your hand on them.
Great question. Here's a lingot. It would be helpful if someone would give an equally great answer. In fact any answer at all would be an improvement. Maybe DL is trying to tell us that using the present tense for a future conditional is idiomatic, but the translation to English in the future tense makes no sense to me while your translation does make sense and it seems ought to be at least accepted as a choice! But DL does not even allow it as a choice. If anyone can give us a convincing reason why carobarro's sensible translation is wrong and would not be understood by a native Italian, that person will also receive a lingot from me.