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This word is used in Arabic before important announcement, to get the attention of the audience. It's like saying to a crowd, "Attention everybody! I have an announcement to make."

In the Quran, it is often used in that sense but sometimes it used for emphasis. When it is used for attention, I'd like to translate it to something the English reader is familiar with, i.e., "Hear ye!" For instance,

What do you folks think? Remember, I always want to match the Arabic word, as it meant in the Seventh Century, as much as possible. Pickthall was fond of the word "Lo!" and it is ideal for this case, but has it become old English hardly used anymore? I think not. People still understand the phrase "Lo and behold!"

But when it is used for emphasis, it is more meaningful to translate it to something like, "Unquestionably", e.g.,

In the first example, the use is for underlining as you point out. In the second case, part of the meaning is contrasting with what had just been said, and "unquestionably" doesn't serve that part well.

I see your point. Should we in such case use something like, "Nay" instead?

Good suggestion. "Nay, it is ...." conveys the meaning to me.