The flaw in your plan is that the 156's ECU can handle up to eight different imobiliser codes, plus you don't code the key, you programme the cars ECU to accept the keys imobiliser code. Whether the keys chip can be over written or not doesn't matter.
Errrrrr. thats where you are 500% wrong.
The cars ecu holds the immobiliser code. YOU CODE THE KEY TO THE CAR.
If you dont believe me phone your local dealer and ask for the immobiliser code for your car. they will ask for your reg or chassis number and give you the number in exchange for 20 quid.
How can they do this. BECAUSE THE CODE IS STORED ON YOUR ECU FROM WHEN IT LEAVES THE FACTORY.
Have you ever tried to code a key to a car using alfa examiner. (obviously not).
You enter the immobiliser code and the car searches for a transponder chip to code to.
Unfortunately you dont code the car to the key.
It is the other way round.
And you cant code a second hand key because the transponder is only designed to be coded once.
The only way to do it with a second hand key is to use a blank transponder chip inside it.
But these are extremely difficult to get hold of.
Probably has something to do with Alfa Romeo quite liking the £200 they make each time someone needs a new key.
Im not getting at you but im trying to keep it factual and what you have just said is missleading and completly wrong.
Someone could buy a second hand key and find out they have completely wasted there money and payed for a key that is useless.
You did get the 8 keys bit right though. ;-)