How can we know in this sentence if they mean, "for the winter" (i.e., for that period of time the season), or "before the winter"?
In speech you can usually tell by the emphasis. Without emphasis on voor it means for. With emphasis it means before. In writing it's less clear, however if it's written as vóór you know there is emphasis on this word, so before is meant.
I know this, but then I'm wondering why the accents are not in this text? It seems a pretty vital thing to know.
It's common not to include these accents for emphasis, usually you'll know the meaning from the context, in Duolingo there is no context, so both meanings are accepted.
Most of the English sentences in this section seem very unnatural and wrong to me, I don't think I would ever use them or hear them from a native speaker.
What context would this be used in Dutch? Could you give me a scenario where it makes sense to use this sentence instead of "will you have come for winter?"
I think they're awkward because they're maybe technically correct, but not how the tense is typically used in English. Future Perfect almost always has some sort of very explicit time marker. This particular sentence could also use a more specific verb, or another word to clarify come, such as home.
"Will you have come before then?" "Will you have visited before Winter has started?" "Will you have returned home before Winter is over, dad?"
And etc, which feel more natural. Maybe it's just that the time marker isn't as necessary in Dutch.
I agree several of the English sentences in this section are unnatural. I've mentioned elsewhere (as have others) that Duo seems inconsistent about allowing "By" at the start of sentences indicating that something occurs by a certain time.date/season, where to me (UK) the "By" would be mandatory. With this particular sentence "Will you be coming for the winter" or "Will you be coming before winter" are both much more natural English sentences. Do either or both capture the original Dutch sentence, and if so can they please be accepted?
I think (not sure) that it's because this sentence doesn't have "hebben" in it, so instead of writing it in full "Will they have come before the winter" they've abbreviated it "They've". Although this makes as much sense as someone asking if you're ok and you reply "Yes I'm"; correct in full but not in abbreviation. A hiccup of whoever translated this and tried to abbreviate it.
That's just my theory at least, I'm probably wrong, because I came to the comments to see if anyone else thought this too.