1. You should be able to create a plugin tied to OnManagerAuthentication with something like this:
If ($user->get('username') != 'admin') {
$modx->sendRedirect('url/to/other/manager/page');
}
You can get the URL by going to the Create/Edit resource page and cutting and pasting the URL from the browser’s address bar.
2. I don’t know of an easy way to do this.
3. You can go to System -> Actions and add one or more custom permissions to each menu choice, then give some users those custom permissions. Important: Be sure to give yourself the custom permissions *before* adding them to the actions.
You might consider just having them work in the front end with NewsPublisher, FrontPage, or one of the other front-end editing extras.