Learn Hardcore Japanese: toutyaku no zikan ga kawatta ka douka, tenin ni kakuninsite kudasai. - Please check with the store clerk whether the arrival time has changed.

の links nouns, so 到着の時間 literally means “the time of arrival.”
到着時間 is a common compound and sounds a bit more concise.
到着時刻 is more precise/formal for “arrival time” (時刻 = specific clock time). All three are acceptable; choose based on formality and precision.

Why is が used after 時間 and not は? Do both work here?

が marks what changed: 時間が変わった focuses on “the time (as the thing that) changed.”
は would topicalize: 到着の時間は変わったかどうか… contrasts this time with others or with a previous topic. Both are grammatical; が is the neutral choice for intransitive change statements.

What does かどうか mean, and can I drop どうか and just say 変わったか?

かどうか = “whether or not.”
With verbs like 確認する, you can often say just 〜か: 変わったか確認してください.
かどうか explicitly includes the “or not” idea and sounds a bit fuller/more careful.

Why does か appear in the middle of the sentence?

Because it’s an embedded yes/no question: [到着の時間が変わったかどうか] を確認する. The か marks the internal question clause before the main verb 確認してください.

Should it be 変わった or 変わりました before か/かどうか?

Use the plain form before か/かどうか: 変わったか(どうか). Don’t use polite 変わりました inside embedded questions.

Why is it 変わった (intransitive) and not 変えた (transitive)?

変わる (intransitive) = “change (by itself).” 時間が変わった = “The time changed.”
変える (transitive) = “change (something).” 時間を変えた = “(Someone) changed the time.” Here we talk about the schedule shifting, so the intransitive is natural.

Could I use 変わっている instead of 変わった? What’s the nuance?

変わった: a change occurred (completed event).
変わっている: is in a changed state now (resulting state). For checking current schedules, 変わっているかどうか is often very natural; 変わったかどうか is also fine.

Why is に used with 店員? Could I use と, で, or から?

店員に確認する: check/confirm with the clerk (target of your inquiry).
店員と would mean “together with the clerk” or mark a quoted content, not the person asked.
店員で (location/instrument) is wrong here.
店員から focuses on the source “from the clerk,” and is more natural with verbs like 聞く: 店員から聞く.

Is 店員 polite enough? Should I say 店員さん or something else?

店員 is neutral; on signs/instructions it’s common.
In everyday speech, 店員さん is friendlier.
Other options by context: スタッフ, 係員 (at stations, venues), or very polite 係の者 (staff member).

How polite is 確認してください? What are more/less polite alternatives?

確認してください: standard polite.
More polite/formal: ご確認ください, 確認をお願いします, ご確認いただけますか.
Softer request: 確認してもらえますか / 確認していただけますか.

Why is there a comma after どうか? Can I move 店員に earlier?

The comma just separates the embedded clause from the main request. You can also write:

到着の時間が変わったかどうか店員に確認してください。
店員に、到着の時間が変わったかどうか確認してください。 All are fine; punctuation and placement are flexible.

Can I say 〜かどうかを確認してください? Is that different?

Yes: 〜かどうかを確認してください is very common and explicit (treats the clause as the direct object with を). Your original version with a comma is equally natural.

What are the readings of the words in this sentence?

到着（とうちゃく）
時間（じかん）
変わった（かわった）
かどうか（かどうか）
店員（てんいん）
確認して（かくにんして）
ください（ください）

Why are there spaces between words? Japanese usually doesn’t have them, right?

Correct—standard Japanese doesn’t use spaces. They’re added here for learners to make segmentation clear. Normally you’d write: 到着の時間が変わったかどうか、店員に確認してください。

Could I omit 到着の? Would 時間 alone be understood?

Sometimes context makes 時間 clear, but 到着の (or 到着時刻) prevents ambiguity, especially if multiple times (departure/arrival) could be in play.

Is 時間 the right word here, or is 時刻 better?

時間 is general “time” and common in speech: 到着の時間.
時刻 is the specific clock time and feels more precise/formal: 到着時刻. Either works; for timetables/announcements, 到着時刻 is often preferred.

Should ください be written in kanji (下さい) or hiragana here?

For the request form after a て-form, use hiragana: ください. The kanji 下さい is mainly for the verb “please give (me)” used on its own.