Hello,
Okay for the first question. This is simple. Some applications allow you to install it for all users while others do not. The ones that do not will only be installed for that user account and will not appear on any other user account. That is why those icons are missing because they do not have access to use that application.
For number 2. I dont quite understand what you are asking. You want to control the icons that are on other user accounts desktop? The only way is from the desktop folder of that user account. If they are not present you would have to drag and drop them on there for them.
For number 3. They are a limited user account. They do not have the rights to do so. Limited user accounts can only do what is assigned by them from the Administrator. Nothing mroe nothing less. So if they can not edit their desktop this is because you did not give them permission to.
Cheers,
Mak