There are at least five different ways of writing きのう in kanji, and they all translate to different words. If you are referring to 昨日 (yesterday), it is an irregular use of the kanji, and could also be pronounced さくじつ.
there are several ways to quote someone, the standard way would be "speakerが「quote」と言いました。" In more casual situations, you could drop と言いました and replace it with a simple って.
No.
きのう is 大和言葉(やまとことば), which has been existed before kanji was introduced.
The sound きのう was applied to the kanji 昨日 which also means "yesterday".
(Or rather, 昨日 was applied to きのう?)
There's no connection in sound between きのう and 昨日.
昨日 has another pronunciation, as nice gaijin san mentioned.
昨日 さくじつ : in this case, 昨=さく and 日=じつ.
It is the same about きょう and 今日.
(今日 can be pronounce こんにち as well, 今=こん 日=にち)
Yes, both きのう for 昨日 and きょう for 今日 are examples of 熟字訓 (じゅくじくん), or kanji compounds used for meaning only. It's the flip side of 当て字 (あてじ), which are kanji used for reading only -- e.g., 寿司 for すし, or 滅茶苦茶 for めちゃくちゃ.
Yes, both きのう for 昨日 and きょう for 今日 are examples of 熟字訓 (じゅくじくん), or kanji compounds used for meaning only. It's the flip side of 当て字 (あてじ), which are kanji used for reading only -- e.g., 寿司 for すし, or 滅茶苦茶 for めちゃくちゃ.
there are several ways to quote someone, the standard way would be "speakerが「quote」と言いました。" In more casual situations, you could drop と言いました and replace it with a simple って.