But my main problem is at the first (lower) step, of how to cut the corners to match the wall!
You can select and adjust the 'boundary' of the stairs (each side) just as you would adjust a polyline or fill.
This will adjust the shape of the stair.
Don't select the stair 'baseline' - use TAB to cycle through multiple selections if you need to.
Adjusting the baseline will affect the construction of the stair (flights and landings).
One of the forum moderators.
Versions 6.5 to 25
Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10
Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10