What is the role of "single" in sentence containing superlative?
"Single" in the superlative expression is just for emphasizing the uniqueness/singularity of whatever you're talking about. There is no semantic difference between "the most + [singular noun]" and "the single most + [singular noun]". Some words like 'sheep', 'fish', or 'deer' represent singular and plural at the same time. By putting 'single' in front of superlative expression, it becomes very clear. For example, "It's the single largest sheep that I've ever seen." The sheep in this sentence is clearly a sheep, not many sheep.