I have read the documentation and I understand that I can get the manager by RadAjaxManager.GetCurrent() method and subscribe for its events. The method will return null if there is no manager on the page, similar to ASP:ScriptManager implementation.
"4) Only one instance of RadAjaxManager control is supported
The previous limitation of single RadAjaxManager per naming container is extended so that now there should be only one instance of the control on the whole page. This means that you cannot have one manager on the main page containing a user control and another inside the user control itself (or Master and content page). Otherwise an error is thrown.
Instead, having a manager instance on the main page/MasterPage, one can already add a RadAjaxManagerProxy in the user control/content page. The Proxy control purpose is similar to the one of ScriptManagerProxy and it could be used to add the necessary settings design-time within the user control/content page, instead of adding them programmatically finding the manager from the main page/master page.
Additionally, if one still needs the manager instance at the code of a WebUserControl for example, she can get the manager by RadAjaxManager.GetCurrent() method call. The method will return null if there is no manager on the page, similar to ASP:ScriptManager implementation."
About your second question, please review this
online demo :)