I'll answer your questions based on MY experience with an Alfa Romeo Guilia

I suspect in your case, you will need to purchase both. The 12+8 connector should
bypass you having to go through the secure gateway module (so you would unplug the
factory OBD2 from that module and connect the 12+8 cable instead and get direct
access like any other car).

The Auto Auth account is a subscription which requires Internet access each time you
start the process to "authorise" your Autel device to be accepted as a genuine factory
scan tool. Note: The dealership will have particulars about You and the serial number
of your Autel device to deny any warranty claims if you mess something up for
warranty purposes (as stored in some online database to say that your tool accessed
this particular VIN at this date and time).

My suggestion to you is to just get the 12+8 cable, and save yourself the lengthy
process if you have multiple of these vehicles to scan again in the future.