A lot of questions! I will give a partial answer to three:
Is residential any more viable or any less viable than other types of practices currently?
What clients are actually hiring LAs as individuals or firms?
What, if any, clients prefer to avoid LAs and why?
Based on my experience, I believe design build firms will continue to dominate the residential markets. I see increasing market share in the commercial sectors too. At some project scales, D/B is just a better business model then the traditional adversarial “professional vs. tradesman” model . Less defensive drawing, more flexibility in implementation, and one direction for the client to point in.
Unfortunately, I see LA missing this boat–too few of us are starting, these types of firms and “landscapers” are filling the void. But they are hiring.