I'm wondering what the reason for the mispronunciation of 円 in English came to be "yen". I can understand how some words like 東京 became "Tokyo", but "en" to "yen" seems strange. On a side note, why is this symbol 「￥」 used to denote currency? And would you pronounce that the same as 円 as well?
Why is it pronounced "yen"?
I was actually wondering this a month or so ago, but found the answer on the Wikipedia article for yen/en.
The spelling and pronunciation "yen" is standard in English. This is because mainly English speakers who visited Japan at the end of the Edo period to the early Meiji period spelled words this way. ... In the 16th century, Japanese /e/(え) and /we/(ゑ) both had been pronounced [je] and Portuguese missionaries had spelled them "ye". Some time thereafter, by the middle of the 18th century, /e/ and /we/ came to be pronounced [e] as in modern Japanese, although some regions retain the [je] pronunciation. Walter Henry Medhurst, who had not come to Japan and met any Japanese, having consulted mainly a Japanese-Dutch dictionary, spelled some "e"s as "ye" in his An English and Japanese, and Japanese and English Vocabulary (1830). In the early Meiji era, James Curtis Hepburn, following Medhurst, spelled all "e"s as "ye" in his A Japanese and English dictionary (1st ed. 1867). That was the first full-scale Japanese-English/English-Japanese dictionary, which had a strong influence on Westerners in Japan and probably prompted the spelling "yen". Hepburn revised most of "ye"s to "e" in the 3rd edition (1886) in order to mirror the contemporary pronunciation, except "yen". This was probably already fixed and has remained so ever since.
The Symbol ¥
As long as you are in Japan, ¥ is pronounced the same as 円. I'm not sure exactly why the both exist, but I'd guess that it's the same reason that we have "dollar" and "$" as well as "euro" and "€" - ¥ is not a kanji or word, but rather a symbol. Similar to these examples, even though ¥ comes before the number, you would still pronounce it at the end of the number; ¥100 and 100円 would both be pronounced ひゃくえん.
To add to the second point to atlantiza answer, note in general that currency symbols have two parallel bars crossing a letter for the main unit and a single bar crossing for the sub unit (Recently, I see only single bar with the dollar symbol, though). In case of American dollar, the main unit uses what looks like "S" today, which is not straightforward, and the sub unit uses "c", which is related to "cent". With Japanese, it is "Y", and should be clear that it comes from "yen".
For asking why "￥" is used, I think you have culture bias. Why is "$" is used instead of writing "dollar"?