Doesn't OpenWatcom mean using some seriously ancient C/C++ language standards? Any idea why it's preferred over gcc?
While old, OW has things like a resource compiler and was more targeted to OS/2. It is also very stable. Documentation is better too. The toolkit also has a lot of documentation and OW can easily use the toolkit.
Though GCC can use some of the OW tools like the resource compiler and even often uses the same linker depending on flags.
OW also supports 16 bit programs, most important for device drivers.
b) If you want to port things from the Linux word you can use "gcc" and some libraries like "Qt5" and SDL.
Nah, I want to learn to do it right!

c) Do you want to create a WPS application.... prepare to suffer

Maybe you should start with PM first and later learn about WPS.
So what's the actual difference here? My understanding was that WPS was just a shell on top of PM, like how in Windows, Explorer is a shell on top of GDI, but I'm getting the impression that's way off!
Well, while the WPS is
just a shell on top of the PM, it is also fully object orientated, eg the Desktop is descended from the folder class, so is basically a folder. Also the media player in AOS (music mostly) is also based on the folder class, a folder with music widgets. You can also use most any language to write for the WPS, rather then being stuck with C++, through idl's and the SOM compiler.
Basically, Explorer is just a really bad copy of the WPS.