And to get those Faces straight, you can for instance:
Select these 4 Vertices, F to fill them with a (temporary) Face and make a new Transform Orientation:

After making the new Orientation, you can Delete that Face (Blender will remember the Orientation).
Select these Faces and make a relevant Vertex the Active one, for instance this corner Vert (and use Active Element as Pivot Point):

Then S, Z, 0.
You’ll still have to fix those insets, but I’m sure you are able to do that.