I tried to get around the same problem with your patch but didn't succeed. Changing with runas to an administrator didn't elevate so that service-installation wasn't allowed. Running the service-installation with shellexecute and parameter "runas" after switching to the administrator worked.
I posted a changed version of InstantSupport.au3 in https://forum.ultravnc.net/viewtopic.php?f=50&t=31167
which also supports deinstallation of the service with the already given credentials when finished.