toilet and shower backing up sewage?

In most states you will have an "assumed lease" if you have been paying rent. Usually anyone paying rent, and sometimes even if they aren't, can only be evicted through the courts. Until then, they have to make any necessary repairs. Send them a letter certified mail informing them of everything that has happened, and email them a copy of the letter and the postal receipt. Then, call your local department of licensing or building codes and schedule an inspection. Most major cities have organizations for tenant's rights. They can walk you through the steps.

The problem might not be yours. Check with your local building code department. Your duplex may share a lateral out to the street with the other side of the duplex. In this case their failing to properly dispose of sewage and clog the line may be causing your problem. As long as the landlord is collecting rent from both of you then there is little you can do. If it were mine I'd check with the neighbor to see if their sewer is backing up too. If you both complain then you might get action a lot faster. Do pay your rent because non payment could put you in small claims court and your sewer still would not be working. Good luck.

The rent you owe has nothing to do with the backed up sewer. Did you even report it to the landlord? You did not say. If you gave the landlord proper notice and he has done nothing in five days, then follow up. Perhaps, your repair got missed. You have to be reasonable about it.

If you don't pay your rent on time, expect to be evicted. Your excuse will not hold up in court.

Just because he doesn't work for him anymore doesn't mean the agreement is no longer valid.