Learn Turkish: Kusursuz bir etkinlik düzenlemek şöyle dursun, büyük bir rezalete sebep olduk. - Let alone organizing a flawless event, we caused a huge disgrace.

Yes, you can use them interchangeably. Both phrases follow a noun or a full infinitive verb (like düzenlemek here) to mean "let alone [doing something]" or "never mind [doing something]". The literal meaning of şöyle dursun is something like "let it stand over there," which perfectly captures the dismissive feeling of pushing the first idea aside.

Why does rezalet take the dative case ending -e to become rezalete?

The compound verb sebep olmak (to cause / to be the reason for) always requires the dative case for the thing being caused. In Turkish, you don't "cause something," you "become a reason TO something," which is why it must be rezalete sebep olduk.

Why is the word order büyük bir rezalete instead of bir büyük rezalete?

In Turkish, when a noun is modified by both a descriptive adjective (like büyük) and the indefinite article (bir), the standard order is Adjective + bir + Noun. You can see this exact same pattern earlier in the sentence with kusursuz bir etkinlik (a flawless event).

How is the word kusursuz (flawless) formed?