What others said: always have a lease agreement in writing in future. Maybe there is a sample form available that reflects your state's law. Also, become familiar with your state's law, especially about what's happening in your particular case with no written agreement.
Having mentioned "written agreement", I don't fully agree with some previous comments about not having any recourse. BUT, to get anything in your favor may be extremely difficult, also depending your state statutes.
A lease agreement does not have to be in writing to be effective (with some exceptions), so you offer the premises, the tenant accepts the offer by paying the rent, voila, valid legal contract.
Problem here are the terms. In Wisconsin, if there is no written lease, the statutes rule the rental world, and they cover quite a bit of standard stuff. So by them paying and you accepting the money, you probably have a rental contract. That going on for a year establishes some ongoing relationship, and if they paid always on a particular day, say the first, it could be argued that this is the established due date for the rent and the type of lease, say a month-to-month base don the payment frequency.
My statutes then also give a minimum time frame for cancellation. e.g. 28 days for MTM, unless agreed otherwise.
I heard/read of some tenants who argued successfully in court that they were NOT late paying rent on the 10th, as it was established that in the past they usually paid on the tenth and it was accepted by the landlord. Therefore the 10th was the established rent due date, despite other written agreements.
This should also work the other way around.
So check your statutes, maybe consult an attorney (who will likely eat up anything you in in court...), and see what the rule book says if there is not written agreement. One year tenancy should be enough to argue something in front of the judge.
And then you could always gamble a bit: On one side, if it fails, you loose $100 or so filing fee and your time (if you go yourself to court). Compare that - after research - to your winning chances and the level of collectability (is that a a word?) of the tenant. Then use simple math or gut feel to make the decision.
Other than that: I never had the problem so far with using the deposit for öast month's rent. But in order to prevent this more, I typically ask for a deposit of 1 month plus $200. I often see big question marks int heir faces, as everybody expects the standard 1 month and cannot understand that it technically is unrelated.
I explain that if they use it for last month's rent, they will likely loose the addl. $200, as I will charge late fees. Also, if I have to evict - and in most cases my tenants are not collectable - I get the month rent, plus court fees, plus some extra for the holes in the wall.
To your question 3:
Short answer: yes.
You have to get the judgement. That shows up in the court records, all public. Then they might not pay. Then you have go to court again based on the first judgement, and they have to answer questions about their finances and work, I think, and then you get the permission to garnish wages, and then you have to start that somehow. I've never done that, my only chance I had, that b.... filed for bankruptcy a month after she moved out. So maybe someone else can chime in.
Credit report: you cannot report yourself, the court doesn't do it, either. You can sell the debt to a collection agency, which can do it, and some organizations where you can be a member can do it, I think.