What Brandon is saying is that borders are safer than an immigration office. At the border he is already leaving, so he pays the fine and keeps walking. At the immigration office what can they do? Sure he pays the fine, but what about him? He can't get an extension can he? There's only one avenue. He will be arrested and escorted to the airport and placed into IDC until he leaves. He will receive a ban and it will probably be five years.
You don't get a visa and it's nothing to do with customs. I think you'll find it's immigration who give a free entry stamp, not a visa. Never mind, you've a lot to learn
ED visa is not Multi-Entry. The point of this visa is to come to Thailand to study. You might be permitted to get a re-entry permit during any period your school closes for holidays etc, but that will be a case by case situation