In order to move the splitter position, you can use the following code snippet: 
radSplitContainer1.MoveSplitter(radSplitContainer1.Splitters[0], RadDirection.Left); 

The splitter position can be restored using the RestoreSplitterPosition method: 
radSplitContainer1.RestoreSplitterPosition(radSplitContainer1.Splitters[0]);

In addition, we introduced a way to change the image of the NextNavigationButton/PrevNavigationButton. Here is the example how to achieve it: 
radSplitContainer1.Splitters[0].NextArrow.Visibility = ElementVisibility.Hidden;
((RadButtonElement)radSplitContainer1.Splitters[0].NextNavigationButton).Image = new Bitmap(Properties.Resources.nextImage);

radSplitContainer1.Splitters[0].PrevArrow.Visibility = ElementVisibility.Hidden;
((RadButtonElement)radSplitContainer1.Splitters[0].PrevNavigationButton).Image = new Bitmap(Properties.Resources.previousImage);

Keep in mind that the image should be set again if split panels are added or removed because the splitters are recreated.