Do your other Steam games work okay? It could be a Steam issue, although looking at the steps you have taken then I doubt it. I know that the firewall can be an issue and/or if you have something like McAfee web advisor installed it can interfere with games although not Mount and Blade as far as I am aware. I don't think that would be causing those problems though.
You could try the fix where you close Steam, delete all folders except the Steamapps folder and the Steam.exe file (be warned, screenshots are usually saved to the userdata one so make sure you are not deleting those), restart steam and will automatically reinstall the files although it will now consider this the first time Steam has been installed on the computer so if you are planning to sell something on the market then be careful. I would not recommend this method unless all hope is lost.
