I haven’t heard much about kashering a backsplash,
Does it need to be kashered (assuming one is not covering and is only kashering)
If it does need kashering does it require the whole height or just the height of your pots?
If it does need do you need to hit every spot with a direct hit?
If so that sounds very difficult especially if you have to avoid outlets.
Kashering a backplash, as you write is difficult, because you need to get the actual stream of water on each part, up until he place where the steam reaches. The way that it can be done is by pouring the water on the bottom parts first and then work your way up. However with electric outlets there, it is not safe. The main things though is to cover the backsplash well.
Have a kasher Pesach