We know that Tommy Lee Wallace doubled for Nick Castle whenever Michael had to perform a stunt. And we definitely know the shot of Laurie getting her arm slashed was done by Wallace.
My question is. Did Tommy Lee Wallace play Michael during the shot of him appearing out of the shadows behind Laurie?
Based on nothing more than the way the mask stretches in that scene, I’d say its likely Nick. At the risk of sounding like a jerk, TLW has a large and distinctly shaped forehead that is really easy to spot under the mask. Nick stretched the mask from ear to ear (his afro) and Tommy stretched it back to front.