One way to handle your default route conflict can be this, as most of the time when you use you route it will hit the action method without the [HttpGet]. All you need to do is a GET, whenever you sort or page the web gird, it will try to get data and hit the a HttpGet Action, this will work as follows:
[HttpGet]
public ActionResult YourActionMethod()
{
return PartialView("YourView",YourModel);
}
The best part is, upon sorting, the request will send a parameter named as "sortBy" too, you can use this here and decide what you want to do with the binded Model with the grid. You can inspect what URL the Sort header will hit by using the "Developer Tools" in your browser. You can use this action do as you will,
Note : By default the action method it would be hitting would be same as the controller name.