1) take a look here: http://www.telerik.com/support/kb/aspnet-ajax/general/using-dynamic-unique-names-for-javascript-functions.aspx.
2) create a unique function that will handle OnClientCLose for each UC instance
3) declare these functions in each user control and wrap them in a RadScriptBlock
this should let yo reference the grid from the user control.
If the window is not in every instance it would be hard to know which grid you need to update. You can set a custom CSS class to the desired grid and use that to access its client-side object:
var grid = $telerik.$(".myCustomClass")[0];
if(grid && grid.control) grid.get_masterTableView().rebind();