I would vent through the attic and out the roof. That's what I'm doing. I had a roofer come in and cut the hole in the roof and install the roof cap. It's really nice because I was venting directly out a window and rain and wind were a real problem. Now, I can work in any weather!
And no, you can't use the same door for fresh air and exhaust unless the openings are ten feet apart.
You could also duct out the door that goes directly outside and use the door to the garage for makeup air. You would have to also open the outside garage door a bit so you have fresh air coming in. Personally, I think it would be tough to vent out a door because it all has to be closed off so the exhaust doesn't come back in.
Also, I would not exhaust through the garage. It needs to go directly outside, and that would be really hard to do going through a door and all the way out the garage to the outside.