I was using linked clone pool and did all 5 things you mentioned. I looked at a log in /root/viewplanner/harness_core/vpresults/; it claimed that the desktop vm was in customizing state. Indeed it was stuck in customizing state when I went back to Horizon connection server.
When I tried to build a linked clone pool with quickprep without any scripts then it was fine and the desktop vm is in available state but it was stuck at customizing if I selected sysprep (autologin and join domain).
If I connect to the desktop clone at vcenter web ui, there is a dialog with message "Windows could not continue with the installation. Restart the computer ...". I clicked the OK button and it restarted. When it came back the auto login working and password was set to Q1w2e3r4! It also joined the domain but it is still in customizing state in Horizon View Server. Run again, same error.
Once again thanks for your response.