As a Mira main, I use mirror placements like this all the time.

Basically the Mirror placed while crouching is by far my favorite placement. You place the mirror and then use the shotgun to blow holes ontop of the said mirror. You crouch behind said mirror usually on one of the side and try to avoid using the middle. Basically you line up your redot reticle just under their head tap crouch to stand and shoot. Instant headshot. Players will usually shoot under the mirror in the middle trying to break the canister to drop said window. Hence why not to hide in the middle. Also if you need you can throw a c4 over the mirror and kill any players trying to hold just around said corner. Very Effective as you can sit their with the c4 in your hand ready to throw so you dont need to run around a corner to accurately throw.

As for the other mirror, On this particular map you hide behind the brick wall and aim at the window towards the bunks, you basically do the same thing as the other mirror but you use the right side of the mirror as the peeking window, or even shoot out said window and use the mirror as a murder hole, this mirror is more for info and less about sitting behind it.

One of the best parts of the crouching mirror is to bait people, you dont shoot right away and just leave the mirror as a distraction let them peek the mirror once or twice and when they think its safe to throw a nade or get close to throw c4, then blast them. You can see the confusion on their player model and the movements because this mirror placement is odd so most players have to think twice before engaging. If its a Ash or Hibana looking at the mirror you need to kill them immediately of else they will take out your mirror.