If you have the ability to conquer Warsaw yourself then do it. As for the nobles not following you maybe it is glitched for you for some reason. I remember what I did for that part of the quest was to just simply build up a lot of money through trade and winning battles. Then I bought a whole bunch of upgraded mercs who then killed many of enemies before going down. It is possible to take it yourself just stay alive and help out in key points of the battle to help sway the tide.
If you lose, then this is where the money comes in to help. ASAP, run to the nearest mercenary camp and again recruit as many as you can. Then attack again immedietly. Even though you loss the first time, it usually means that they are a bit weeaker now and a second attack will secure your victory.
BTW, if yuou need anymore help on the quest or in the game you can PM (personal Message) me if youd like.
Hope this helps