Apologies for the delay. I had to step away from the desk for a bit.
First of all, you should contact the debt collection company. Say that you're completely disputing the debt, and that they should reflect that in any and all reports that they make about it, as well as contacting the landlord.
Next, you should bring up the fact that you were constructively evicted from your apartment. Explain the smell and other conditions that were caused by the landlord, and that you have witnesses to. Explain that you did not have an operational bathroom, and that the leaks persisted. After numerous opportunities to repair, the landlord did not, and the inaction by the landlord amounted to a breach of the "covenant of quiet enjoyment" that landlords have with their tenants.
Explain that you were forced out of your apartment and that you DID incur costs as a result of these actions, and that if the landlord wishes to pursue you for rent, you're more than willing to countersue for wrongful eviction and the expenses that you incurred as a result of being forced to move out.
Ultimately they might determine that there's little upside to bringing a case against you. The best that they could seek is to go to court and to try to get a judgment against you.