If I attack a decoy dragon, and my opponent uses its effect to try and special summon a seven star and up Dragon, can I use my DD Crow to remove the dragon, and what happens afterwards???

We played it if they chose the dragon, I removed with D.D. Crow. A replay does not occur, so no monster was summoned and decoy dragon was left to be targeted and destroyed. Does it work like that or because the monster I was going to attack technically never appear so my attack gets negated? Or do I have to reattack decoy dragon anyways?

A replay will not occur and you will continue attacking Decoy Dragon. You do not have to "reattack" Decoy Dragon, and the attack is not negated.