The short answer is that you cannot use 'at all' in a simple affirmative declarative sentence. You can use these: They will not finish it at all. Will they finish it at all?
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moon7296Your example 'if they finish it at all, it will be a miracle' can be similar to 'if they finish it somehow, it will be a miracle'?Because you left out, "...it will be a miracle" it reads rather odd.
But as you said, at all cannot be used in a affirmative declarative sentence like my example 'they will finish it at all.'
moon7296if they finish it at all, it will be a miracle~ If they succeed in finishing it, which seems highly unlikely, it will be a miracle.
moon7296if they finish it somehow, it will be a miracle'~ If they find a way to finish it, and thereby finish it, it will be a miracle.