I'd suggest you contact support with this one.
With 7.5 I always instruct my consultants to uninstall this first with the original 7.5 installer and only once uninstalled then you can use the v7.6 or later installer. Once you are on the 7.6 or later new installer technology an in-place install of the later version will work well but not for anything pre 7.6 as just too much changed in the installer technology.
I would suggest you need to get to a state where you can install 7.5 again cleanly and then uninstall it again, but for that you'll need the original media too. There were some issues with the installer in the 7.5.x timeframe where it wouldn't uninstall cleanly if you tried to uninstall with a different AD user to that who originally installed it but there is a script/util that can fix this but only works if you run it BEFORE you try to uninstall.
I think you can probably rename an "Install" folder in the programdata folder under the Touchpaper root folder and this will allow you to reinstall 7.5 cleanly again without trying to restart from where it failed getting you into that endless loop.