There are actually two answers to number one.
1A. The same reasons human beings don't get melted when struck by lightning IRL. Lightning strikes so quickly, it doesn't last enough to melt human beings, but the heat is still felt. This basically means lightning bolts created by the Goro Goro no Mi can hurt Akainu, but not vaporize him.
1B. The Gomu Gomu no Mi offers complete immunity to the Goro Goro no Mi and it's properties when the two interact directly. What this means is, the consumer of the Gomu Gomu no Mi is COMPLETELY immune to the Goro Goro no Mi's effects when the lightning touches the Gomu consumer's body directly, including the heat. This, however, can be circumvented by using a medium. Heat, being a form of energy, is never lost or created, but transferred, so if the Goro Goro no Mi heats up something, such as Enel's golden staff, then the Gomu consumer can be affected by it because the heat of the Goro Goro no Mi was transferred into the gold, making it the gold's heat now, which Luffy isn't immune to.
2. That makes sense.
3. The Hie Hie no Mi can reach below zero temperatures in an instant. Add into the sheer volume of ice that can be created, the ice would snuff out the fire by dropping to temperature too low for ice to be sustained. It doesn't need to freeze the fire.