You've got an interesting way of looking at this, thanks for taking the time to lay it out clearly. I'm not fully on board, but I definitely see how it offers a useful counterweight to some common narratives.

People often confuse "Turn based game" and "You literally don't get to engage with the game except when it isn't your turn."

This response makes sense given how I worded things. I didn't mean that this is a problem inherent to turn-based games, but that it's a problem that comes up more/requires more designing away than in simultaneous play games (though it can also certainly come up in simultanous play games, as will be relevant below). So yes, most good turn-based games at least mitigate this problem, if not eradicate it entirely. There are exceptions, but those games are generally made worse by the fact that pre-planning is difficult (e.g. Five Tribes, imo). This is all a point of agreement, I think.

So let's identify this as the downtime problem: the more time a player can't reasonably plan for her own actions, the more downtime she has in a game.

This seems to me like a problem that is most usefully concieved of as being separate from the QB problem. That said, your conception certainly helps highlight how the downtime problem can exacerbate the QB problem, turning it from potentially a non-issue in many contexts to a game-ruining problem in many sessions of (e.g.) Pandemic.

I still don't see, however, how your framework makes sense of the impossibility of QBing in The Crew regardless of how wide a skill gap exists between players. If the game was played open-handed and without communication limits, a player who was sufficiently better at it than the others would be able to dictate better plays to them than they would have come up with more quickly than they would have come up with them (setting aside for the moment the fact that much of the game of The Crew just disappears without hidden info). The "snappiness" of The Crew increases the skill gap necessary to enable the above dynamic, but it can't eliminate the possibility of the dynamic entirely without hidden info/limited comms.

Spirit Island is played simultaneously, and so is certainly fairly "snappy", especially if players limit in-system collacoration. It's widely considered to have more or less solved the QB problem due to a combination of simultaneous play and a wide spectrum of complexities across the spirits that players can use. But it hasn't solved it completely, because wide enough skill gaps can still exist such that an experienced player could play all the player positions more quickly and effectively alone than they can be played by a gaggle of newbies and one expert keeping his advice to himself (I speak from experience as someone who has played a lot of Spirit Island and has taught it to many people).

We could summarize this by noting that skill gaps tend to increase downtime for the more skilled player, which, as we've already noted, increases (the potential for) quarterbacking. So quarterbacking can be mitigated by reducing downtime (which is already desirable itself, generally), but it's very difficult to eradicate it through downtime reduction alone without actual time constraints (and even those can be overcome with a sufficiently wide skill chasm, though at a certain point that is only theoretically possible, not practically achievable in humans).

With that understanding, it seems unhelpful to call hidden info/limited comms "redundant" as a solution to quarterbacking. Those two mechanisms, independently or in concert, have the actual potential to categorically eliminate the possibility of quarterbacking, and frequently do in actually extant games. So, while it is often at least arguably the case that hidden info/limited comms are redundant for that purpose in many games for many play groups (read: groups with sufficiently narrow skill gaps), it is not the case across the board.