Learn Hardcore Korean: yeonhyu donganeun gyohwan·hwanburi jiyeondoel su isseoyo. - During the holidays, exchanges and refunds can be delayed.

은 is the topic marker. 연휴 동안은 means “as for during the holiday period,” subtly contrasting that time with other times. You could say 연휴 동안 without a particle; it’s grammatical and neutral. Adding 은 makes the “holiday period” the topic and slightly emphasizes it.

What’s the difference between 동안은, 동안에는, and 동안에?

동안은: topic-marked “during that period,” with contrast/emphasis.
동안에는: time marker 에
- topic 는; very common and natural in notices: “during that period (as for).”
동안에: plain “during,” no topicalization.
All are correct; the choice affects nuance more than meaning.

What does the middle dot in 교환·환불 mean? How do you read it?

The middle dot (·, 가운뎃점) links parallel items, like “and/or,” similar to a slash (/) in English. Read it aloud as 교환과 환불 or 교환 및 환불. It’s common in signs and notices to group related items neatly.

Could I replace the dot with other connectors?

Yes:

교환과 환불 (and)
교환 및 환불 (and; formal)
교환/환불 (and/or; informal writing)
They’re all natural in this context.