If the management company is not responding, then you may want to communicate directly with the attorney.  They are sending you demand letters based on incomplete and inaccurate information.  This tends to agitate some attorneys when their name is on the letter head.  He may actually be more helpful in getting movement and resolution.  Make sure that you only provide copies of your paperwork because you don't want your documents "accidentally" getting lost.  Whether submitting info to the attorney or the management company, make sure you do so certified mail so that you have proof they received the information--- because right now, many things will be a he-said/she-said situation.  If you're not comfortable talking to the attorney on your own, consider hiring an attorney for the limited purpose of submitting a response to his letter and your request to add your girlfriend.  Before you talk to either attorney, take the time to make a timeline of what efforts you have made so far, the associated documents, and who you talked to each time.  As you go forward, update your list as you talk anyone associated with this process.