It appears that reprogram initialisation can be done in software:
_reboot_Teensyduino_();
I am guessing: if the security byte is set and the program button is inaccessible, calling the above command in my sketch should put the teensy into program mode ready for a new sketch upload.
...This is a hardware enforced security feature. It is designed at the hardware level inside the chip to have no way to cheat. The only way to unlock security is a full erase of all flash.
I have read it and I think that you need to saw the chip to reach the individual modules to read the flash.You simply might want to read the reference manual to reach your reliable conclusion.
Paul answered this above, too.
If you do not believe Paul's answer, why will you believe other's?I still would be very grateful if somebody with more experience and competence than I could confirm my assumption (or not).
... Regarding your other question "without opening physically the main processor chip", I do not know what sorts of reverse engineering techniques exist (if any) for these chips. I do not personally know of any exploits or weaknesses. But I can not comment on the design of the chip.