Apple warned you. This printed little guide is probably still in your iPhone box. Everything is right in there.
Okay, perhaps English is your second language?
You constantly seem to confuse terms like "MAY cause damage" or "MAY void" to mean "WILL" do so. You also confuse "SHOULD use authorized" with MUST. The first forms are allowable by law. The second demand forms are normally not.
It did caused damage, as evidently your phone is bricked now by error 53.
Backwards. The phone was not damaged until it was bricked. But I can see Apple's lawyers arguing your way.
Apple should never allow 3rd party Touch ID sensor without proper encryption keys to access the A7 chips encrypted storage. If Apple allows this, it would bring serious mayhem to not only Apple device security but also Apple Pay security and credibility.
Just to be clear: the sensor doesn't access anything. It's an input device, like a mouse. Your mouse cannot access your hard drive. Nor can the sensor access the secure enclave.
The sensor simply sends fingerprint data over a serial line to the CPU, which then forwards the data to the secure enclave to look at and possibly authenticate against registered prints in its secure storage.
Sensor data -> CPU -> secure enclave -> authentication Y/N flag back to -> CPU
That's all. So no, the sensor cannot attack the rest of the phone directly. The security hole being presented is that if a evil sensor knows your real fingerprint, it can duplicate it at some other time for someone else. Of course, a much cheaper (and easier to do without being detected) fake finger also works, especially if you can follow and target someone.
I wonder if next Apple will brick laptops if you change the stock mouse or keyboard. After all, a malicious third party mouse or keyboard could remember your movements / keystrokes, and thus are also security risks.
You missed the bit where an unauthorised modification, e.g. not following the due process of key pairing, invalidates your warranty, in the same way as rooting the OS does.
One of the possible arguments that Apple lawyers will use. But you know what? Sometimes that which is legal is still the wrong thing to do, both from moral and PR standpoints.