if you have an irp file then you should be able to get the ism file as well. Unfortunately the way the process works is this: we compile the irp file to the ism file and then this is compiled to the MSI file. You can compile the inc file into an ism ...
I understand your pain with regards to the manager. We have however provided a version and instructions to install without this functionality. Please check http://support.installshield.com/kb/view.asp?articleid=Q111107
Are you uninstalling from add/remove programs? If you are then I would recommend rerunning the original installation and this should allow you to uninstall successfully.