[procaryote]

This requires the device manufacturer to have the capability to decrypt the data (to be able to do so when all this process is properly observed)

If they have the capability to decrypt the data, a court can compel them to do so, disregarding the process you suggest. A cyberattack could achieve it without a court order.

This can't be solved technically.

[wulfstan]

It requires decryption by means beyond the sole supply of a user-owned key. That doesn't require a manufacturer to be capable of decrypting it.

I suspect that there are many ways that can be achieved, all technical ;-)

[procaryote]

describe one.