Learn Hardcore Japanese: muda ni matu zikan wo herasu tame, seki wo yoyakusita. - To reduce wasted time waiting, I reserved seats.

無駄 is a na-adjective. Adding に turns it into an adverb, so 無駄に means “wastefully/needlessly,” modifying the verb 待つ. So 無駄に待つ時間 = “time (that one) waits needlessly.”
Contrast: 無駄な時間 (adjective + noun) = “wasted/useless time.”

Is 待つ時間 natural, or should I say 待ち時間?

Both are grammatical, but 待ち時間 is the standard, set expression for “waiting time.”

待つ時間 literally means “time to wait / time (that you) wait,” and is fine.
A very natural rewrite is: 無駄な待ち時間を減らすため(に)、席を予約した。

Is 無駄に待つ時間 the best way to say this, or is 無駄な待ち時間 better?

Both are correct, but they frame things differently:

無駄に待つ時間 focuses on the action: “time you wait needlessly.”
無駄な待ち時間 labels the time itself as wasteful and is more idiomatic in everyday Japanese. Most natives would prefer 無駄な待ち時間 here.

Why is it 減らす and not 減る?

減らす is the transitive verb “to reduce (something),” which you use when you actively reduce an object (here, 時間).
減る is intransitive, “(something) decreases.”
So: 時間を減らす (you reduce the time) vs. 時間が減る (the time decreases).

Should it be 減らすために instead of 減らすため? Is the に optional?

For purpose, ために is the canonical form. However, in speech and casual writing, the に is often dropped, especially before a comma: …ため、….
In careful or formal writing, keep に: 減らすために、席を予約した。

Does ため(に) here mean “in order to” or “because of”? How can I tell?

Here it means “in order to” (purpose): the subject intentionally reserved a seat to achieve the goal of reducing waiting time.
As a rough guide:

Purpose: volitional action in the main clause by the same subject (e.g., …ため(に)、席を予約した).
Cause: non-volitional result or explicit reason (e.g., 雨が降ったため、試合は中止になった = because it rained…).

Why is it 席を予約した with を? Could I say 席の予約をした?

席を予約した treats 席 as the direct object: “I reserved a seat.”
席の予約をした nominalizes and emphasizes the act: “I made a reservation for a seat.” Both are fine; the first is more direct and common.

Is 予約した plain form okay? What about 予約しました or 予約しておいた?

予約した = plain past (neutral/informal narrative).
予約しました = polite past; use with strangers, customers, etc.
予約しておいた adds 〜ておく “do in advance (for later convenience),” which fits well here: 待ち時間を減らすために、席を予約しておいた。

Can I say 待つの時間?

No. Don’t insert の there. To modify a noun with a verb phrase, use the dictionary form directly: 待つ時間 (“time to wait”).
Alternatives: 待っている時間 (time spent waiting), or the set noun 待ち時間.

How do you read the sentence?

むだにまつじかんをへらすため、せきをよやくした。
Kanji readings: 無駄(むだ)、待(ま)つ、時間(じかん)、減(へ)らす、ため、席(せき)、予約(よやく)

Can I move 無駄に elsewhere? What if I say 無駄に減らす?

Placement matters. 無駄に currently modifies 待つ (wait needlessly).
If you say 無駄に減らす, it modifies 減らす and means “to reduce wastefully/pointlessly,” which is not the intended meaning.

Could I use ように instead of ため(に)?

Yes, to express prevention or the desired state:

無駄に待たないように、席を予約した。 (so that I won’t have to wait needlessly)
待ち時間が長くならないように、席を予約した。
ため(に) states a concrete purpose; ように often frames avoiding an outcome or ensuring a condition.

Is there another verb that works well with time besides 減らす?

Yes:

短くする (make shorter): 待ち時間を短くするために…
短縮する (to shorten; formal): 待ち時間を短縮するために…
減らす is perfectly fine and common too.

Should I use 席, テーブル, or 座席 when reserving?

Restaurants: 席 or テーブル (e.g., テーブルを予約する).
Trains/planes/events with assigned seating: 座席 (e.g., 座席を予約する／指定する).
Choose based on the context.

What’s the difference between 待つ時間 and 待っている時間?

待つ時間 = general/expected time to wait (more abstract).
待っている時間 = the time actually spent while in the state of waiting (progressive nuance).
Either can work; pick the one that matches your focus.

Could I rephrase with passive to stress “being made to wait”?

Yes. For emphasis on external causes:

無駄に待たされる時間を減らすために、席を予約した。
This highlights “time I’m made to wait (by someone/something)” rather than your own waiting.