SUMo Portable leaves a Registry Value behind in the HKCU\Software\KC Softwares Registry Key, causing that value and consequently that key to be left behind after.
Adding the following fixes the bug.
[RegistryValueBackupDelete] 1=HKCU\Software\KC Softwares\SUMo
I missed this earlier but this will be adding handling to the launcher in the next SUMo release. The dev itself has removed the bug as well, so it's fixed in the current release.
Sometimes, the impossible can become possible, if you're awesome!