I do not find this likely. We know this was true with Vega, and that they signed both ends of the firmware. If you stop signing one end, as we know they have (the one responsible for volts and clocks no less), it is indicative you have stopped all together.
Not necessarily, as I wrote before the BIOS has different regions with some protected and some not. Critical parts like the Secure Boot UEFI GOP are most likely signed while parts modifiable by the AIB OEMs are not. I suspect that AMD has their own internal versions of tools like RBE aimed directly for consumption by OEMs. This makes sense because otherwise the chip would have to do one of those things:
1) only store the AMD public key and AMD would have to sign
every single bios for every AIB card variant separately - that's a huge amount of useless work
2) contain a set of public keys for usage by OEMs - a waste of space, would have to contain every future OEM key at the time of chip's production
3) have some mechanism for one-time programming of the OEM key - difficult to do, risks bricking the chip on failed program, no real benefit so far since critical parts are already protected
This is speculation of course, but it mirrors how motherboard BIOSes are constructed with most using 3).
As generous as EU warranties are vs stupid things like not fixing a card if a sticker has fallen off a screw (because that by itself is not "proof of misuse"), they certainly cannot force a manufacturer to repair if the manufacturer can actually prove that it was broken via deliberately tampering (eg, flashing a non standard VBIOS then borking the card in the process of reverting it back that leaves part of the modded BIOS in EEPROM is about as obvious as it gets).
The manufacturer would have to prove that what the consumer did with software actually managed to damage the hardware and cause the issue. I've yet to hear of any manufacturer claiming this with computer hardware. This is purely economics - it'd take more time and thus money to prove the consumer wrong than to just replace/repair the part.
You're assuming the courier understands exactly what he's looking at or hasn't been instructed by the courier company to not get involved in Ebay disputes.
Not necessary, the courier can take photos to attach to the report, for example. The shipping company then vouches to Ebay that the photo content was what was in the box. It's standard procedure.
Or that you're at home when the courier delivers and someone else in the household just signs for it.
This is considered the fault of the receiving party. Tho not adhering to shipping company procedures, most couriers will allow you to call them in cases like those to "fix" a report even if he wasn't physically there. I know it's shady from the courier, but that's the reality of how they work. The safest route with hardware is to make sure you open the box with the courier present.
And for delivery drivers unfamiliar with PC hardware, "I've no idea what that is, a bit for a robot maybe?" (in response to seeing a GTX 750 in a RTX 3090 box) doesn't insure against anything. The manufacturers can't do a thing vs not receiving correct item whilst returning it to Ebay sender only works if you complain you got the right item but it was faulty, not that you got an entirely different item. There is no real protection against the fraud I mentioned which is exactly why Ebay are siding with the scammers in such incidents.
I'm pretty sure that if a seller did this multiple times they'd get banned/investigated by Ebay. There's always a risk when buying hardware on auction services.