[Nabeshin]

He could still claim you gave him those instructions, and face some harassment from him in case thing go bad, I wouldn't risk it

Isn't the outcome the same as when you fake sign and then claim DNA?

Could be, but in that case it's the courier's fault for handing it to someone else and/or not requiring you to sign it personally, in our case you told him it was fine to act this way and so the fault is on you

Beside that, is any case known were the Steam deck was locked through Valve? Is there a possibility of them being able to lock it? I was thinking that as soon as you're login in with your account, they might flag ist and/or disable internet access.

Definitely order it from a different account than the one you plan to use it with, they can't ban you for purchasing and using a device that you had no way to know it came from a SE