why not just use the upper right wall as both a bearing & shear wall?

Yes. We can use that. What if we cannot use that wall?Lets say wall length in right side is too small. How can we add the shear wall?

One of two scenarios:

1) You're using full height studs because there's no floor diaphragm there to brace them, so your shear wall is two stories high.

2) You design a wind girt to support the wall at what would otherwise be a hinge.

In both cases, you need to get load out of the second floor diaphragm and into your walls. This will happen with a collector. It's typically the the rim board or the top plate of the lower wall. So the diaphragm transfer in that area to the upper right has to be sufficient to dump the load into that collector, and then the collector has to deliver it to that first floor shear wall. Presumably the roof diaphragm load is going straight through the stacked walls.

Then, if you want to really design it properly, you need to consider sub/transfer diaphragms to get the load from the the main diaphragm out past the re-entrant corner formed by the stair.

You can always use the bottom wall of the stairs as a shear wall. That's what I would do to sleep better and it is hard to mess up by the contractor. That wall should be 1 story high.

For footings of interior shear walls I generally put a grade beam. This provides mass for the overturning resistance, adequate anchorage for hold downs.

Depending on the overturning and HD forces I might look at a stemwall with cont. spread footing.

Thank you all