I have a slightly convoluted one.
I control a
Meandering Towershell and a
Clever Impersonator copying the Towershell. My opponent controlls a
Colossus of Akros (It's
Pacified or something). I attack with my two turtles, exiling them. During my next declare attackers step, the turtle and the impersonator enter the battlefield attacking. I can't copy my towershell with the impersonator because they enter the battlefield at the same time, so I copy my opponent's colossus this time.
1) Is this how this situation happens?
2) If a creature with defender enters the battlefield attacking, does it still attack or does defender cancel it out?
1) Not quite. You're right about
Clever Impersonator not being able to copy something that's entering the battlefield simultaneously with it, but in this case,
Meandering Towershell sets up a delayed triggered ability, so those will both trigger at once, go on the stack in the order you choose, and then resolve one at a time. If you put
Clever Impersonator's delayed trigger on the stack first, then it will enter the battlefield second and you will be able to copy the Towershell. However, you might still want to copy the Colossus, because...
2) Defender prevents you from declaring an attacker, but it doesn't inherently prevent the creature from being in the "attacking" state, so if you do somehow put a defender onto the battlefield attacking, it will continue through combat like any other attacking creature.