It is my belief that SDL is cleaner to look at and code with than win32.

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1) Is SDL compatible with Mac (along with the given that it is on Windows)?

It is compatabile with both macOS and linux, along with a load of other platforms:

Quote:Source: SDL_ website

SDL supports Linux, Windows, Windows CE, BeOS, MacOS, Mac OS X, FreeBSD, NetBSD, OpenBSD, BSD/OS, Solaris, IRIX, and QNX. The code contains support for AmigaOS, Dreamcast, Atari, AIX, OSF/Tru64, RISC OS, SymbianOS, and OS/2, but these are not officially supported.

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2) To make the application run on Mac OS, would I have to avoid using particular header files?

Yeah, any windows only ones. But using pure SDL you will have little use for them for a tetris program.

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3) Which of the two would prove more useful for the gaming industry (SDL or Win32, or both)?

Both, and others. But win32 code is needed to interface with the OS for things other than just making a window and drawing to it. SDL has been used in commercial games (ports of commercial games usually )

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4) Which has cleaner code structure? Or does that really just depend on the programmer?

I believe SDL does, but its personal. I just don't like win32's naming conventions.

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5) I've heard SDL is quicker to code with, is this true? (not that it matters, but I was just curious if it was true or false)

Well, not really. After a couple of programs you have a bunch of code you can use with in others that can handle a lot of what you need. So not particularly.

But code to get as simple SDL window up and running s very quick to write and easy. So learning may be faster, but YMMV

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6) Would the switch be worth it (Win32 -> SDL)

IMO yes.

But again, YMMV.

use sdl, easier + much faster to get up + running thus more time to do what u want (develop a game) its cross platform as well (mac yes)
also win32 is becoming obsolete (like my knowledge of cobol has become)