You are certified once you complete the Police Academy, once you are employed and sworn in, then you are a Police Officer. You can self sponsor or you can go to the PS you want to work and see if they are hiring.
You would need to have written permission from the owner. If you don't have that you would definately get some tickets. I can't see an owner of a property giving you permission because of the liability.
Every agency will have be different. However smoking alot for two years will not help you. There will be alot of candidates that will be picked way before you that will have no drug history.
Why volunteer as firefighter? Do you want to be a fireman or a police officer? If you want to be a police officer go and see if you can volunteer at your local police department.