"There is no money."
From my understanding, が is used with existential phrases, like this as well as rhetorical questions, which this also might be. お金がありません (Money, there is not).
金よう日 : The Japanese word for Friday literally has the Kanji for Gold/Money in it (きん).
Why is "Money" called おかね on it's own? Is it Kinyomi and Onyomi again? Please clarify!
Try to think of Kanji in regards to what word it is forming. So, rather than say "Money is called かね on its own," it is much more accurate to say "金 is pronounced as かね when used for the word money."
In Japanese, Kanji technically doesn't mean anything until it is used in a word. Your best bet is to remember each pronunctiation as words, and learn the kanji from that, i.e. from words you already know. This is a lot easier than learning a kanji and saying "right, this one is pronounced like this, that and this."
Yes. Usually(!!!), onyomi is used in compound words (2+ kanji) with no okurigana (attached hiragana), and kunyomi in all other cases. Note that the word "金曜日", while it is okay to spell it (partially) using hiragana, does not actually contain okurigana.
I just assume that vocal Japanese and writing Japanese are different languages and don't mind reading Kanji this or that way.