I just wanted to offer a simple theory for what happened to Stamets in the mirror. My initial reaction watching the scene is quite simple - it was our first glimpse showing him out of sync with time. "Mirror" Stamets does the same actions as "real" Stamets - just a few seconds later. It's a subtle foreshadowing of what we find out about is life out of the timeline in "Magic to Make" and also his sight into different points in time "Si Vis Pacem, Para Bellum". I think the personality changes can wholey be explained by the DNA upgrade and spore drive interaction. For me, this simple explanation keeps me believing in the choices the writers are making for Stamets.
Here is my review/reflection on episode one of Picard and also a little bit about why I love TNG so much. Let me know your thoughts on the series so far and your hopes for the rest of the season. https://www.claytemplemedia.com/post/star-trek-picard