sounds to me that you have the prob figured out. chances are you can order the replacement spring and detent direct from Walther. for peade of mind and safety consideration, I would order both parts and not just the spring, ( the spring  appeears to be the prob) also when you have it apart I would dress up the detent divot on the safety lever also and dremel the spot a bit deeper but not wider. This will give it more of a bite  

"I would dress up the detent divot on the safety lever also and dremel the spot a bit deeper but not wider. "

The detent doesn't hit the bottom of the 'orange-slice' recess in the lever. increase the width also, just a hair, with your Dremel.