Glad all worked out for you and all went home!
We have your post and what you faced then we have this story in this post Officer successully DATD outside his home Both of you faced a life threatening situation I have to ask what is the difference in the two situations? Yes one is a knife and the other is a gun but you both are looking death in the face. What I see is training when you are staring at death you act you don't do "show and tell".
At 15 feet with open knife in hand both you and your brother in law could be cut before you cleared leather by a good knife man. What I see at the pushing of the button and the knife springing opening most likely there would have been shots fired shortly thereafter. I'm sure there are those here who think I am a trigger happy SOB but give me a break I'm an old man who doesn't question what you have on your mind, I'll shoot. I've found over the years if it looks like a duck it usually is.
I see you needing more training so you know just what you did face in that situation.
Again glad it worked out and you went home