Here's another interesting piece of software that's been forgotten, Apple Remote Desktop. It's actually sort of painful to look at on a retina display.
Apple does offer enterprise management. You might want to look at getting a mac mini with OS X Server (they have neutered the latest version however)
http://www.apple.com/osx/server/
Like chewy said though Apple Remote Desktop will do most of what you are looking for.
Also Macs definitely CAN be managed in large environments and done more very easily for less cost as well. Windows machines you have to pay for per unit licensing if you're using any of their enterprise software. Apple's software you don't.
See this link.
Remote does guis too!
e.g. windows: https://support.microsoft.com/en-us/instantanswers/ff521c86-2803-4bc0-a5da-7df445788eb9/how-to-use-remote-desktop
e.g. mac: http://www.apple.com/remotedesktop/
e.g. linux: Xvnc (less likely, given there's no photoshop for linux)
shrug depends on your environment and management tool of choice.
At worst you should have ARD on hand (http://www.apple.com/remotedesktop/). At best your management tool of choice will have mechanisms for making automated processes.
I rely heavily on self service in my environment. Just kind of put it all out there, users can pick and choose what is applicable to their environment. Some things I automatically push out, flash updates, etc, certain configuration profiles (login window, screen locking, etc..)
Deploy studio is a solid option for imaging. If you are looking for more granular scripting control and/or long term management solutions check out ARD or JAMF's Casper suite.
I know Apple Remote Desktop has a 'Curtain Mode' that is "For those times when you want your actions to be hidden from the end user, there’s Curtain Mode. This allows you to block the view behind a virtual curtain, while retaining full control of the screen. It’s perfect for those working on public-facing systems."
http://www.apple.com/remotedesktop/newfeatures.html
I know it's not free, but I figure it's a place to start searching.
Is there a reason you're not using Apple's own Apple Remote Desktop? I've been using it for some time to manage Mac clients (though it is sometimes sketchy across a VPN), and it's performed well for me. Sounds like it can do all the things you're looking for in the OP.