Thanks for the research! If they're signing the ECID using an interactive signature algorithm, then it sounds like they've thought it through pretty well.

> And I think the IMEI is more commonly used to identify the radio, not the device itself.

Across manufactures, I'm not sure another quasi-unique identifier in common use.

> Every time you update an iPhone it generates a nonce, called APTicket. Apple signs that, your ECID and the firmware.

This is one variant of interactive signature scheme.