It helps to think of hard drives same as any other computer. When you boot Windows, for example, after the computer reads the boot sector, NTLDR is used to launch it, which loads NTOSKRNL.EXE that runs the show. Well, it's a bit more complicated than that, but good enough for now. Can you wipe the boot sector or NTLDR and expect bad things not to happen?

1. Technically always, but critically during drive startup. It may be possible to start a drive with hot swap with a foreign SA in case the original SA is not readable at boot. Emphasis is on "may" and that's the basis of the "hot swap" technique. This becomes increasingly difficult with modern drives as manufacturers shift to storing unique parameters on the PCB in NVRAM. See next question for further explanation.
2. The firmware is loaded into the drive's RAM from the SA and keeps running in RAM. That's why you can start some drives with a "loader" in RAM when the SA is inaccessible. However, drive may need access to SA for other reasons.
3. For all practical purposes, since the beginning. The early drives relied on an external controller. IDE is "Integrated device electronics" and comes with a built-in controller. The reason why we can sometimes make a malfunctioning drive identify itself in BIOS when it's set to Slave but not as Master, is because the Slave drive completely bypasses its own IDE controller and uses the one from its Master. IDE is also known as EIDE and now ATA. SCSI is a different story.
4. No.
5. This is not usually a defect caused by user actions, although rapid powerup/powerdown and continuous thrashing can contribute to the problem. Unstable power is a proven drive killer. Bad sectors developed in SA will cause SA problems.
6. Varies and really not important.
7. The location varies between manufacturers. You need special tools to access it and those tools generally know where it is. Physically, it is often either at the edge or in the center. Then again, you also sometimes have extra copies elsewhere.

I don't claim to be an expert guru.

Doomer knows more about drives and how they work internally than you can imagine. Take his answers. The problem with being an expert is that there are too many ways to answer a question, especially when they contradict each other. I have that problem with MCP exams "I know the real answer is X which can vary with Service Pack W, but they don't expect me to know Y and are probably basing this on Service Pack U, so the answer they are expecting is probably Z". That's why he requested clarification, I believe.

I would like to suggest that this thread be moved to Hard Drive Technology subforum.

Thank you very much for your insight,

bitor