Learn German: Der Snack ist so knusprig, dass wir ihn alle mögen. - The snack is so crispy that we all like it.

In German, pronouns must match the gender of the noun they replace. Since the word for snack is masculine (der Snack), you have to use the masculine pronoun. Here it is the direct object (the thing being liked), so it takes the accusative form ihn instead of the neutral es.

What does the so ... dass structure do in this sentence?

The combination of so and dass is used to express a consequence. You state a quality in the first part (so knusprig), and the dass clause tells you the result of that quality (that we all like it).

Why is the verb mögen at the very end of the sentence?

The word dass is a subordinating conjunction. This means it introduces a dependent clause and forces the conjugated verb, which is mögen here, to move to the very end of that clause.

Could I say dass wir alle ihn mögen instead of dass wir ihn alle mögen?

You might be understood, but dass wir ihn alle mögen is the standard word order. In German, short pronoun objects like ihn generally prefer to sit right next to the subject (wir), which pushes words like alle further back in the sentence.