Learn Hardcore Japanese: meiwaku wo kaketakunaino ni, kyuuyou de yoteidoori ni ikenakatta. - I didn’t want to trouble anyone, but because of urgent business I couldn’t go as planned.

彼に迷惑をかけた = I caused trouble to him.
みんなに迷惑をかけたくない = I don’t want to trouble everyone. A related expression is 迷惑になる (“to be a nuisance”), which describes something being troublesome in general, not someone causing it. Polite set phrase: ご迷惑をおかけして (I caused you trouble…).

Why is it かけたくない and not かけないたい?

The “want to” form is verb stem + たい; the negative is たくない.

かける → stem かけ → かけたい (want to cause)
Negative: かけたくない (don’t want to cause) Past negative: かけたくなかった.

What does のに mean here? How is it different from けど or が?

A のに B = “even though A, B,” expressing a strong contradiction, regret, or frustration.
けど/が give a milder “but/though,” often just linking clauses or softening a statement.
Your sentence with のに sounds more regretful than with けど.

What is the の in のに? Do I need な with nouns or na-adjectives?

のに is a fixed conjunction meaning “although.”

Verbs and i-adjectives attach directly: 行くのに, 高いのに.
Nouns and na-adjectives take なのに: 学生なのに, 静かなのに.

Why 急用で? Could I say 急用があって or 急用ができて?

急用で uses で to mark cause/reason: “because of an urgent matter.”
Alternatives:

急用があって = there was an urgent matter.
急用ができて = something urgent came up (more spontaneous). More formal: 急用のため.

What does 予定通りに mean, and can I drop に?

予定通りに means “as planned/on schedule.” The に makes it adverbial (“in the planned way”).
You can drop に: 予定どおり行けなかった. Both are common; に can feel a touch more explicit/formal.

Is 予定通り spelled 予定どおり? How should I pronounce it?

Both 予定通り and 予定どおり are standard. Pronunciation is よていどおり. Many style guides prefer writing どおり in kana (especially in this “as X” sense) to avoid confusion with 通り meaning “street.”

Why is 行く in potential form (行けなかった) here?

The potential form of 行く is 行ける (e-row base), so past negative is 行けなかった = “couldn’t go.”
Note: 行けない (can’t go) is different from the grammar 〜てはいけない (must not).

What’s the difference between 行けなかった and 行かなかった?

行けなかった = couldn’t go (inability due to circumstances).
行かなかった = didn’t go (you didn’t go, often implying choice).
In an apology with a reason like 急用, 行けなかった fits best.

Can I say 行くことができなかった instead of 行けなかった?

Yes. 行くことができなかった is a bit more formal/literary and slightly heavier. In everyday speech, the potential form 行けなかった is more natural.

Should it be かけたくなかったのに (past) instead of かけたくないのに?

Both work, with a nuance difference:

かけたくないのに: a general, ongoing desire (“I don’t want to cause trouble (in general), but …”).
かけたくなかったのに: focuses on that specific occasion in the past.

How can I say this more politely when apologizing to someone I was going to visit?

In the base sentence, the person troubled is implied (often the listener). To be explicit and polite:

ご迷惑をおかけして申し訳ありません。急用ができて、予定どおりには伺えませんでした。
Here 伺う is the humble form of “go/visit.” You can also say 申し訳ございません for extra formality.

I’ve seen のに mean “for (doing something),” like 包丁は野菜を切るのに使います. Is that the same のに?

Different use:

[purpose] のに = “for doing …” (nominalized clause + に as a purpose marker).
のに (conjunction) = “although/even though.”
Your sentence uses the “although” のに.

Can I emphasize “not as planned” with は, like 予定通りには行けなかった?

Yes. 予定通りには行けなかった adds contrastive は, emphasizing that it was specifically “not as planned,” often implying something else did happen or contrasting with expectations.

Can I flip the clauses: 急用で予定通りに行けなかったのに、迷惑をかけたくない?

That would mean “Even though I couldn’t go as planned, I don’t want to cause trouble,” which feels illogical in context. Keep the intention or expectation first, then the contrary result: 迷惑をかけたくないのに、…行けなかった.
If you just want to apologize without the contrast, you can drop のに: 急用で予定どおりに行けず、ご迷惑をおかけしました.

Why are there two に in the sentence?

They’re unrelated:

のに = the conjunction “although.”
予定通りに = adverbial に meaning “in the manner of/as planned.”