Thanks @syman2000 , @RDAOU I was able to make both methods you suggested work.
@syman2000, the Join Geometry method worked. At first I couldn't, but after another day of experimentation I finally found the "Golden Path" for joining walls:
1. Un-join the foundation and brick walls
2. Join the Geometry of the brick with the foundation below (@syman2000 's method only)
3. Set the heights at the connection
4. Re-join the brick walls
5. Toggle the brick Wall Join
6. Align the end of the top foundation wall with the side of the other foundation wall
7. Drag the handle of the other foundation wall to just touch the first one
8. Done! 🤯
@RDAOU, the above sequence is also required for the Create Parts method, with the addition of Creating the Parts of the foundation wall step in the middle. You need to join the brick walls before joining the foundation walls.
One thing I noticed, in both methods, you end up with two separate join conditions: one for the brick wall join and one for the foundation wall join. But if you join the foundation first, then the brick wall, there is only one join condition (as indicated by the square around the intersection with the Wall Join tool).
Rant: Why are wall joins in Revit so painful? Why can't they just give us control instead of trying to hold our hand. We're the ones doing the design, I think we can handle it.
Mike