Ok-- that makes sense. If you were able to swap a 2017 transmission into a 2014 with no unaccounted for connectors, it makes sense that it would be an external computer that puts it into the Ds mode.
Still curious as to what happens in the transmission once the message is sent by the computer. The Ds mode is different from the L mode, and each of the modes could be effectuated by the use of different mappings of belt ratios at the pullies. But I wonder if either mode is effectuated (totally or in part) by the auxiliary transmission.
You should do that-- I wouldn't have hesitated at all to tear into it! I've already seen a number of tear-down videos of the JF015E, and it's all surprisingly straightforward.
I somehow had it in my mind that the belt system in a CVT would require hyper-exact settings/tolerances, similar to how pinion gears have to be precisely matched in a differential.
But watching those videos, there was nothing special about any part of that transmission. Pretty simple to get out of the car, to take apart, to determine what the problem is, and to fix it.
Since I've not done it, I can't be absolutely certain, but I'm pretty sure that if I had a transmission problem, I could not only fix it myself, I could effectively remanufacture it myself. The result would be a transmission that's as good as new and ready for another ~200K miles of service.
I suppose I could be proven wrong on that, but at the moment I at least wouldn't be afraid to try it.