Author here. Good points you made here. As I mentioned before, I can't see there being any reason why the agent couldn't charge at least some fee to the company as well (which would be greatly reduced from what recruiters are charging now, so their cost to hire would be lower).

I'm not sure what a company would think. If it were to become more common, it would just be a way of doing business. I negotiate most of my candidates' salaries and packages now anyway, so having an intermediary would not be a huge leap for most companies that already use recruiters. The leap would be the term 'agent', but I think companies would respect the fact that someone takes their career seriously to have an advisor. We'll see.