It depends on the dialect. The standard pronunciation is an English v sound, but many people use a sound between v and f. In my accent in the west of the country it's almost an f in many words, so the number 4 (vier) is pronounced the same as the English word fear. To be safe, always use a normal v in the middle of a word but feel free to use a more f like sound at the beginning of words.
Good question. Ask the Englsih language ;) you need a time machine though (and the ability to personify english)
All jokes aside I háve read why english ended up with this uncommon construction, but unfortunately forgot again. (I think some people were claiming it celtic in origin and others saying it had to do with the Dutch predicative present participle (if that's the correct term)
In colliquial speech the n at words ending in en sort of fades away. If you talk formal, slow and articulated it is there. But for casual conversation it's usually just there in its eroded form. Which depending on speaker can go from slightly less audible to nearly completely missing.