"I need to apply a weight of 1kg (Fz = -9.81N) at a point on the top surface 90mm from the clamp, or 100mm from the origin. But I can’t seem to find a combination of distance/face that works. I either get no moment, if I choose face_0 in red above, or, seemingly the wrong moment in that the strain messurements are off by 3 times if I chose face_1 as below. "
So if I understand correctly you are applying a remote force to the blue point on the far right in your image and mapping that force to either the small top surface or the large top surface. When you apply a remote force that force is tied to every node on the surface you are selecting. So, essentially you are creating a distributed load on the surface. You are not creating a point load on the surface and this would explain the small strains compared to a beam in pure bending.
To get what I think you are wanting, one approach is to go into Onshape and create a small surface centered at 90mm and then apply the load to that surface. You can do this using the Split command that you used previously. Just make a rectangle or circle a few mm by a few mm and that should work for you.
Another option is to cut your beam at 90mm, the extra length you have does not affect the bending of the beam. Then in Simscale you can either apply a force to the edge or the end face.
I hope this helps. Please let e know if you have any other questions.