It will be similar to military basic training. A lot of physical training, weapons training, tests. You will get yelled at for alot of different reasons.
I don't think you should worry too much. It might not be a big issue, make sure you disclose it on your background. They will definately ask you alot questions about it, so be ready for it.
ATF Police work the crimes related to laws governing alcohol, tobacco and firearms. They also get involved when there is an arson investigation and bombs.
Alcohol, Tobacco and Firearms
Honestly, yes it could hurt your chances. Right now there alot of people that want to become a police officer, if they have a clean record they will get hired first.
You are going to need at least three to five years experience on the street as a patrol officer. You then wait for a position to open up and then you have be selected for canine.
Each states has very specific laws pertaining to spring assisted knives. You would need to contact local PD, they will be more than happy to assist you.