Her explanation of the problem was nonsense.
無意味 in this sentence doesn’t function as “nonsense” but “ineffective” or “not fruitful”. 彼女の説明は無意味だった is more likely to be interpreted as this way: Her explanation was actually insightful and made a lot of sense, but it was just too late to change the decision made by her boss.
The English sentence can be translated in Japanese as:
的外れ consists of 的（target）and 外れる（to be diverted）. It’s like an amateur archery game. 的外れな as an adjective is used when someone says completely irrational, or off-topic.
意味不明な means “other people cannot understand the meaning of what you are talking about, and they are appalled at you”. So, it’s pretty much the same as “nonsense”.