Learn German: Ist der Athlet so stark, dass er jeden Wettkampf gewinnt? - Is the athlete so strong that he wins every competition?

By placing so directly in front of the adjective stark, the sentence emphasizes the degree of his strength: "Is the athlete so strong that...". If you used sodass as one word, it would sound like a more general consequence: "Ist der Athlet stark, sodass..." (Is the athlete strong, so that...).

The whole sentence is a question. Why doesn't the second half have a question word order?

The main clause, Ist der Athlet so stark, already makes the sentence a question by putting the verb first. The second half is a subordinate clause introduced by dass. Subordinate clauses always send their conjugated verb (gewinnt) to the very end, regardless of whether the overall sentence is a statement or a question.

Why is it jeden Wettkampf instead of jeder Wettkampf?

Wettkampf (competition) is a masculine noun (der Wettkampf). Because it is the direct object of the verb gewinnt (it is the thing being won), it must be in the accusative case. For masculine nouns, the accusative case turns jeder into jeden.

Is the comma before dass mandatory?