Why can't the camera in video mode switch back and forth between Front Camera and Selfie mode? This is very frustrating.

Do you mean while you are filming?  The Apple Camera app won't even let you do that.  The only way I know to accomplish this is Filmic Pro's DoubleTake app.  It allows you to film from a front and selfie camera simultaneously.  If you want to switch from one to the other, you have to do it in post with an edit.

Here's an example of the use of DoubleTake with a front facing + selfie shot synced up in one of my videos at 3:13.  Its a little shaky as it was filmed before I bought my gimbal:

Hi there. Thank you for reaching out. You cannot switch between DJI Osmo Mobile series front and back cameras when recording. When recording, the Front/Back Camera Switch is dark and cannot be touched. Thank you for your understanding.

You can not do it during the recording...Try to use one angle at a time