Understood - but the offline device does have the private key and presumably could display that, and if it can do that then it could also display the "k" value that could then be audited via another offline device.
Yes, but that only shifts the trust from one offline device to the next. You have to know that any of them is properly doing their job. Looking whether the offline verification device is doing its job is not any easier than looking whether the offline wallet does implement ECDSA according to the standards.