"likely that many of the EV-to-grid arrangements will be for fleet vehicles"
Wont these be out and about during the day when the sun is shining and need to be charged during the night? Seems like these are the opposite of storage, they would exacerbate the problem? Creating a lot of demand(lots of buses in lots of cities) charging at night, and driving during the day.
"will be technically sophisticated"
This is an assumption, though it does seem technically possible.
"scenario you describe where a private EV will have it's battery depleted"
Im an environmentalist myself, but if I wake up on the morning of my vacation, and my car is at < 100%, I'm not going to be happy. You might say you could provide a way to signal that your car needs to be at 100%, but I see a non trivial amount of people enabling that all the time.
I guess if we require every parking lot with > x spaces to have plugs for all vehicles to charge/store during the day. Still requires people to actually bother plugging them in. Of course, they would also have to plug them in during the night, and unless they are getting paid to, I dont see people going through the minor hassle if it is no benefit to them. I'm just not really optimistic about the vehicles as storage idea, sadly.
Sorry for the pessimism, but I feel like there is too much naive hand waving among environmentalists. I think we need a more realistic, critical discussion, focused on likely, probable solutions. Theres too much: "Everybody can just go out of their way too do x." Theres still people who think putting an aluminum can in a different bin is too much work. We have to work with these unfortunate realities, not just dismiss them.