It should not be a problem to buy a car in Canada and drive it in Washington State If you want to later register it in the U.S. You will have to do a lot of paperwork though.
yes
Yes
no
A Canadian Citizen can not drive a car with US plates into Canada. I'm not sure why, only that its a Canada rule.
Yes
12
18 to register.
You can buy a car under your small business. You have to register the car and insurance under the name of your business.
yes in some countries
You can not register or title a car in a child's name in this state. I do not know the current age limit. At one time it was 21.
If you live in Illinois you can buy a car in Wisconsin. You may have to pay the tax difference when you register the car in the state you live in.