For the most part there are plenty of hard working and honest contractors in the United States.
However, being one of the most lucrative businesses, there are plenty of room for the scam artist and hacks. I'm sure thousands get taken every year. That is why people are willing to pay more for a contractor that has a good reputation.
I think you should contact your local Building Official at you City or Town Hall and explain your situation. But be prepared.
There was probably no building/electrical permit issued and someone will have to step up to the plate for that. It is up to the home owners to make sure you have them.
But, more importantly, you need to get the electrical work inspected.
Also, if your contractor did the electrical work and is not a licensed electrician, the State will want to have a chat with him as well.
If in your state, contractors are required to be licensed, you may be able to file a complaint with that licensing board.
Be forewarned, the only things this inspector will be looking for is building code violations. The nails and the mess in the garage, well that is something you will have to deal with.