The implementation only works for a RadPropertyGrid with a hidden toolbar. As soon as you set RadPropertyGrid.ToolBarVisible=true, the coordinates returned by member Bounds of all MyPropertyGrid...AccessibilityObject's are wrong. Thus wrong items are highlighted. I haven't found a solution for this problem yet.
Then there are some minor correctable errors in code. For example you should replace the  methods GetFocused() and GetSelected() in class MyPropertyGridCellAccessibleObject by the following implementation:

        [SecurityPermission(SecurityAction.Demand, Flags = SecurityPermissionFlag.UnmanagedCode)]
        public override AccessibleObject GetFocused()
        {
            Console.WriteLine("GetFocused()");

            MyPropertyGridGroupAccessibleObject group = this.parent as MyPropertyGridGroupAccessibleObject;

            if (group != null && ((MyPropertyGridGroupAccessibleObject)this.parent).Group == group.Control.SelectedGridItem)
            {
                return this;
            }

            MyPropertyGridItemAccessibleObject item = this.parent as MyPropertyGridItemAccessibleObject;

            if (item != null && ((MyPropertyGridItemAccessibleObject)this.parent).Item == item.Control.SelectedGridItem)
            {
                return this;
            }

            return null;
        }

        [SecurityPermission(SecurityAction.Demand, Flags = SecurityPermissionFlag.UnmanagedCode)]
        public override AccessibleObject GetSelected()
        {
            MyPropertyGridGroupAccessibleObject group = this.parent as MyPropertyGridGroupAccessibleObject;

            if (group != null && ((MyPropertyGridGroupAccessibleObject)this.parent).Group == group.Control.SelectedGridItem)
            {
                Console.WriteLine("GetSelected() - Group " + this);
                return this;
            }


            MyPropertyGridItemAccessibleObject item = this.parent as MyPropertyGridItemAccessibleObject;
            if (item != null && ((MyPropertyGridItemAccessibleObject)this.parent).Item == item.Control.SelectedGridItem)
            {
                Console.WriteLine("GetSelected() - Item " + this);
                return this;
            }

            Console.WriteLine("GetSelected() - null");
            return null;
        }

And you should avoid using a null-object-reference by inserting
if (null != parentItemElement)
and 
if (null != parentGroupElement)
in the implemenation of the Bounds-member of same class, after these objects are assigned,