I have some questions.
I don't understand why 悪夢 is read as ゆめ not あくむ.
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That's a technique Japanese authors use every so often: playing with furigana (rubi) glosses. You can put a different word in the gloss and in the kanji, and in this way insert two words simultaneously at the same position. The implication is something like: the title is pronounced yume "dream", but when saying "dream" they mean a nightmare.
悪夢 should be read as あくむ, and it means a nightmare. "夢" ipse reads ゆめ, and it means a dream. 夢 (dream / hope) and 悪夢 (nightmare) are different words.
"ゆめ" added to 悪夢 as a ふりがな (vocal notation) is 当て字, an arbitrary coinage of the author. It has no currency. You'd better not to follow / emulate it.
http://togetter.com/li/834249 「宝貝人間」と書いて何と読む？『当て字・当て読み 漢字表現辞典』のカオスっぷりが話題
当て字 is an older term.
Perhaps, the newer term 当て読み is more likely to refer to the playful or newly-invented Ateji.
Conceptually related to https://en.wikipedia.org/wiki/Portmanteau words.