I do not understand what problem you are anticipating with hand detection.
How short is the no driver detected timer?
If the drivers hands are properly on the wheel before the turn started why should detection be an issue during the short time the turn is being made?
I'm not sure if your concern is that generally drivers in the current crop of videos are not keeping their hands on the wheel as much as they should, or if you are worried about putting this out to the general public (or even a larger set of testers) that will result in more accidents due to not enough hands on the wheel.
If your concern is the first situation, then, yes, I agree that Tesla should perhaps communicate to that small group that things are not as good as they seem and they should be on a hair trigger to take over at any moment. I do see the urge to do a "look, ma, no hands" when making videos so that it is clear the car is doing the driving. But this is a small group and both needs can be met with sufficient care.
On the other hand, if your concern is the second situation, then there really is no amount of training or disclaimer screens or phone calls from Tesla that can prevent the larger public from going hands off as much as possible. Therefore, you have to design in a system that verifies a certain level of interaction of the driver with the environment to give the driver and car a reasonable chance at preventing any dangerous unforeseen circumstances.
In this case, Tesla has limited options available in the current car models. There is the steering wheel torque sensor we are all aware of. The problem with this, is that it is an indirect way to monitor driver engagement. This can be seen by the number of people working to defeat this sensor by hanging oranges and weights off the steering wheel. This sensor becomes less effective when the car is operating on city streets where large turns by the steering wheel are frequent occurrences. For sure, the driver can let the wheel slide though their hands while the turn is being made, but using the torque sensor during that maneuver would be quite difficult. If the driver puts too much pressure on the steering wheel, it will disengage in the middle of a turn, too little, and the car will have to break off the turn in the middle due to non-detected driver. There are many other circumstances that can be imagined that will cause similar problems with that sensor. Of course, you could limit the nags to just when the car is going straight, but that is pretty much the condition we have right now, so that clearly doesn't solve your "hands off wheel" concerns.
The only other sensor that is available is the interior camera, which is currently only available on Model 3s and Ys. That potentially could solve the problem, but there is some doubt if the camera is the correct place for that role, and/or if the camera is sensitive enough to allow for that type of monitoring. In any case, that sensor is not currently active and in use, so unless Tesla activates it for this purpose, it is not available.
This is why I am currently in the camp that says that Level 3 autonomous driving is a fools errand, and it is better to just go right to Level 4 where you don't need to count on the driver for anything. That said, I would personally be happy with a Level 3 car, and would put up with the inconvenience until such a time that wasn't necessary. I am not convinced, however, the effort required to get a properly working Level 3 is worth the investment on the way to Level 4 (i.e. Level 3 is not a required step on the progression from Level 2 to Level 4). All that driver monitoring that is required for "high level" Level 2 and "basic" Level 3 is a bunch of work that is completely unnecessary if you can get a reliable Level 4 vehicle.
