I don't usually grow sunflowers but are there any leaves below the snapping point? If so you have a chance the plant will branch from there and although it won't grow as tall you will get at least one more flower. Sow more seeds though, as you can do that until the end of May.

Sometimes. What may happen is that is dies, grows just leaves and no flowers, grows multiple but small flowers or grows normally. You could try and root the damaged stalk in water. Good luck.

I would sow another one in a pot as a potential replacement. The broken stem will be a disfigurement if it does continue to grow.