The Building Codes requires that all inside locks can be unlocked with a thumb turn and not a key. Your windows should also open and have locks on them as well. It is a safety issue and should be addressed by the landlord. Your building department will surely be interested in that situation. The Attorney General's Office has a department for landlord-tenant relations.
Most large landlords check credit as part of their regular proceedure.
It is a good idea for you to be up-front and honest with your real estate agent on this point. Not all landlords they represent require credit checks. A good agent may be able to explain away your blemish on the credit report as well. For example the agent can highlight the great landlord references and minimize your past credit problems. You should also contact the credit agencies and get a copy of your credit report .