How to disable screensaver for newly created user?
Have you tried running it as the other user using:

sudo -u NewUser defaults write /Users/NewUser/Library/Preferences/ByHost/com.apple.screensaver.MAC_UUID.plist idleTime 0


The command I found for setting the screensaver to never start was:

sudo -u NewUser defaults -currentHost write com.apple.screensaver idleTime 0