3. Does the controller work on the EmulationStation home screen? I’ve not used the iPac 2 so I can’t help you with any specifics where. However have you read this thread? Alternatively, a temporary solution might be using a different controller. I’ve got a couple CSL gamepads, they’re not ideal for MAME and arcade stuff but you can assign the buttons however you like.
4. Do the ROMs boot? If so, have you setup you controller to take credits? I was stuck trying to use the default keys for ages, for which I suggest reading this.
I hope something in that helps you get your RetroPie running, but come back and post what’s not working.