2. Edit the Profile (C12x25) so the origin is at the top left corner of the profile: just select all the Profile elements and drag. If this profile is used elsewhere in the project, make a copy and edit the copy so the other C12x25 profiles are not changed, then use the copy for the stair.
Just makes me wonder what the various options for alignment in the Settings are actually doing...[Origin, Top Left, Top Middle, Top Right], because if nothing else the selected Profile was oriented such that "Top" would be as expected (ie, the profile wasn't upside down in its drawn definition).
mac ACv27/3001, US full, Sonoma 14.0, 2020 iMac/2023 MacBook Pro
I took a brief look at the GDL code, but as is typical for Graphisoft objects, it involves macros so searching the code is a chore. I think what would be better is to have 9 options (center, 4 corners, 4 mid points of sides) for the origin point for placement in the stair. At least it should work out of the box with the template Profiles, like the C Steel Section.