Made the same mistake. I guess it's this thing in DL. Sometimes you have to answer with a literal translation, even if this is sometimes not very correct English, in order to stress something. Sometimes they expect you to answer in common used language. Never really know what they except. IMHO both should be accepted since we're here to learn Spanish, not correct English!
"Tonight" is always used to mean the current day's nighttime, save for very lyrical or poetic expressions, or to heavily emphasize (this night may be your last!) (This night will be unlike any other). When speaking about the weather, "this night" will never be the translation.
No, it's actually a sound that doesn't exist in English, that is somewhere between a "v" and a "b." (It's called a bilabial fricative.)
To an English ear, it can sometimes sound more like a "v" and other times more like a "b."
Just be aware that there is no distinction between the Spanish letters "b" and "v."