I read that you can disable tr069 (cwmp) from the GUI and from the CLI, to prenvent any unwanted upgrade. Anyway i also read
here that the old firmware is kept in one of the 2 "banks"
so you can reboot your modem with the previous firmware (press the reset button while rebooting).
Even after a firmware upgrade the root password doesn't change and SSH is still active (That's what i understood), only the webui will change. You can upload the unlocked webui again.
Also i don't think TIM will upgrade any modem outside of its ip range (just my opinion, i can be wrong)...unless the modem check itself for an update ?