My questions are hypothetical, but hope I am giving all the info needed to answer them.
Have not worked with MS Anti XSS before, but if it works flawlessly with the MS AJAX Toolkit, it should work with the telerik Ajax controls, too. With regards to the WCAG compliance, I know a page on the telerik site which presents what level of compliance their controls support - view it here: http://www.telerik.com/products/aspnet-ajax/getting-started/tech-sheets-overview/accessibility-support.aspx