Bumpy, Another reason I didn't make a new door, I don't have the machine or cutters (or access to em) to shape the stiles, rails and raised panels to match the existing doors. I just have a Shop Smith table saw & hand sanders.
Carb, Yes the 1st step does slide under the the riser for the 2nd step. No it does not have a tendency to want to flip up. It's a snug fit under the riser, in fact I had to half lap joint the edge of the platform so it would slide under it. The bottom of the both step platforms, there are runners along each side of where it sits on the riser to help keep it from moving.
Milo