GridView - How to specify custom EditFormTemplate / SetEditFormTemplateContent from Controller

Hello Theodoros,
Thank you for the detailed information about your issue. To implement the required functionality, you may pass an HtmlHelper object to some static method that creates GridView settings with templates. Usually, Extension approaches are used in this scenario.
I have modified the GridView - Inline Editing (Templates) demo to demonstrate the required results.
At first, you may create some class to get GridView settings shared between Views.

C#
public static class GridViewHelper {  
   public static GridViewSettings GetInlineEditingSettings(this HtmlHelper htmlHelper){  
      GridViewSettings settings = new GridViewSettings();  
      settings.Name = "gvEditingWithTemplate";  
      settings.KeyFieldName = "ProductID";  
      ...  
      settings.SetEditFormTemplateContent(c => {  
         var product = htmlHelper.ViewData["EditableProduct"] != null ? htmlHelper.ViewData["EditableProduct"] : c.DataItem;  
            htmlHelper.ViewContext.Writer.Write(  
               "<div class=\"edit_form\">" +  
               "<div class=\"line\">"  
             );  
             htmlHelper.DevExpress().Label(  
                edtSettings => {  
                   edtSettings.ControlStyle.CssClass = "label";  
                   edtSettings.Text = "Product Name:";  
                   edtSettings.AssociatedControlName = "ProductName";  
                 }).Render();  
             htmlHelper.DevExpress().TextBox(  
                edtSettings => {  
                   edtSettings.Name = "ProductName";  
                   edtSettings.ControlStyle.CssClass = "editor";  
                   edtSettings.ShowModelErrors = true;  
              }).Bind(DataBinder.Eval(product, "ProductName")).Render();  
              htmlHelper.ViewContext.Writer.Write(  
                 "</div>" +  
                 "<div class=\"line\">"  
              );  
              ...  
            });  
            return settings;  
        }  
    }  

After writing methods to get the settings, you may use this class in various Partial Views as your needs dictate.

C#
@{  
    var grid = Html.DevExpress().GridView(Html.GetInlineEditingSettings());  

    if(ViewData["EditError"] != null){  
        grid.SetEditErrorText((string)ViewData["EditError"]);  
    }  
}  
@grid.Bind(Model).GetHtml()  

I have also created a new suggestion to improve our documentation and describe this approach with code samples: Documentation - Describe the process of initializing a setting object with a template within a Controller and passing this object to a View. You will be automatically notified when the suggestion status has been changed.
Updated by Mike (DevExpress Support):
We have described this solution in greater detail in our documentation at:
Templates
The main idea is to implement the extention method for the Html helper in order to access it in the Controller (not a View) scope.