Ctrl-Alt-Del generates a non maskable interrupt. Yes it was there before Windows, and even before DOS. If an OS wants to react to it other than instantly rebooting, said OS needs to install an interrupt handler for it. That interrupt handler is fired at a way lower level than ordinary keystrokes, malware, or friendly userspace applications.
Bullshit, to the hardware it's just another key combination. It's the BIOS that rebooted the computer when it's pressed. And if you don't use a US QWERTY keyboard, you have to replace the keyboard driver, and this driver will handle the reboot
Using Ctrl-Alt-Del to trigger login gives you two kinds of security:
1. Software cannot simulate a Ctrl-Alt-Del in order to play games with the login screen.
2. By first pressing Ctrl-Alt-Del, the user logging on can be quite sure that they are giving their login credentials to a genuine Windows (or whatever OS) login screen, and not some malware that merely resembles the login screen.
And since they wrote the keyboard driver, they could have done that with any arbitrary combination...