I love everything about your discussion.
1. 100% agree
2. I've thought a little bit about this. I think it would be fine if the warranty can't be voided if the consumer can show in court that there's a preponderance of evidence that the issue they're having is unrelated to the third-party repair. This might mean in practice warranties hard to enforce if you've gone in for third-party repair, but I feel like usually it's old devices that you're fixing with third-parties anyway. It's also possible there's a perverse incentive to make components very sensitive to the case being opened or something, but it seems unlikely that that's possible without making the device fragile overall in situations where it would still be covered by warranty, which would cost the manufacturer money and also make the device less popular in the first place.
Another thing I was thinking, either instead of or supplemental to such a policy, is to require manufacturers of a certain size to have an open certification program that third-party repair shops can participate in, and to honor warrantees if repair has only been done by certified shops. This could be abused if the manufacturer makes the certification program really really shitty so no one does it, and if repairs by the manufacturer themselves is a huge revenue stream then that would almost certainly what would happen; but someone like Apple makes most their money selling new devices (I assume), so why risk the fines from regulators and the bad PR? And so while Apple won't have much incentive to make it easy to get certified, the fact that they have to honor warrantees of devices repaired by certified shops means they'll at least be incentivized to teach the certified shops not to break the devices.
3. I'm skeptical of the importance of this. The security should be in the entropy of my fingerprint, not the right fingerprint scanner being used, no? Like how locking down which keyboard are allowed to be connected to your laptop is not important to securing the password lock?
Spyware on the touch ID sensor is worrisome, but no more than spyware on any of the other input sensors, and is there really anything the manufacturer can do, technically rather than legally, about someone who physical accesses to your device and then hands it back to you to put your fingerprint and password back into?
4. This seems to me like a lack of information problem and an incentives structure problem. In theory, if everyone had total information and knew which of the alternatives lasted the longest and therefore had the lowest total-cost-of-owernship, and some kind of financial engineering let them buy the product on lease or something so the upfront cost could be competitive with the cheaper, shorter-lived alternatives, then everyone would do that, right?
The main theoretical problem here is designing the "lease" to incentivize the consumer to take good care of the product, without being predatory; if we had that, we wouldn't even need total information because it could be viable for the vendor to provide the product on lease or something with a low upfront cost.
Obviously there's a lot of other practical problems like, people who buy new smartphones usually don't want it to last 10 years, they want the latest and greatest cutting edge device; but that seems like it could maybe be solved by saying, you pay $200 now and give us the phone back in 2 years; if you don't give it back then, in 2 years we charge you another $400 (or something?); and then reselling the refurbished phone?
There's also the fact that lots of people who buy iPhones specifically like it in part for the status symbol, so they're not looking for the lowest upfront cost. But the idea that the right pricing structure should theoretically let a company that makes a long-lasting, repairable device with a low total-cost-of-ownership be competitive seems like it should be possible, no?