In your video you are no longer hiding mesh, so that’s why shadow isn’t showing that bug.
You might want to open a new thread on forums or here on AnswerHub for how to switch from third to first person and back, as there are a number of possibilities there and I’m sure community has some answers for you. problem you have with S turning your character around is that controls are still set for third person, in which character can run towards camera rather than run backwards. You’ll need to switch to a control setup more suitable for a first person perspective.
You might consider setting up a second Player Controller Class with controls similar to those you can find in First Person template. Then you can cast to your Game Mode and set Player Controller Class when appropriate.
Hope that helps!