The fingerprint of a public key is defined in BIP 32 as the first 4 bytes of the RIPEMD160 hash of the SHA256 hash of the public key. The fingerprint of a particular xpub can be gotten from that xpub, but this is not useful information.
What the Coldcard wants is the fingerprint of the master xpub. It wants the fingerprint of the key at derivation path m. ...
What are the last bytes? (0a794dec)
In the next operation (HMAC-SHA512) I used the chain code obtained, the key and the next depth (80000002), but I'm not sure.
Yep, that's it : I can see you also prepanded the 0x00 (because serialized privkeys are 32 bytes while serialized compressed public keys are 33 bytes).
I got a 64 bytes result: ...