I had installed mailmerge reports module and it was working fine. We moved to 7.1.15 and had problems upgrading. Now we tried to uninstall the module but it gets stuck at 18% and does nothing. We have tried to reinstall but it shows as installed but looks like it is not installed.
Best try to work out why it is not uninstalling. check the php logs and the sugarcrm.log file to see if there is anything which could cause the issue.
Uninstalling a module manually depends on what files it installs in the first place. Is it possible the answer is Yes, do you want to do it chances are No. however if you still have the package if you unzip it within there should be a manifest.php which contains the files which were copied/modified. which is the files you need to change. It woud not be recommended to do it this way however it could be done. if you want to do it this way i would suggest doing it on a test instance first and things can go wrong very easily doing that.