This is true and the tenant usually wins these sorts of things, but I would say you not paying your last months rent IS a pretty good reason. At the end of the day, if you lived there that month, you owe him that money.
Your landlord/agent does sound poor and the rest of your complaints sound very fair and would be very good grounds to complain as long as you have evidence that you did at the time of the problems and gave the agents/LL time to deal with them.
Just trying to play devils advocate