1. Arctool can create and read a 12 GiB zip file, although it takes more than 4 times as long
It reads the ZIP file (I have done it with 60 GB files), just as fast. What takes the time, is formatting the screen output, so the user can see, and select, the files that are inside the zip file. Overall, much easier to use, but slower.
2. what are the URLs for the zip and unzip builds that support > 4GiB in OS/2?
I don't know, offhand. I use Arca Noae Package Manager (ANPM) to manage that stuff, so I don't need to do it manually. I expect that they are wherever they post the RPM packages. Whether they will work properly, without all of the other stuff that ANPM (RPM/YUM) installs, I don't care to know.
4. Several of the other compression tools, e.g., gzip, xz, are intended to compress in place rather than to build a compressed archive.
That doesn't make sense. How can you compress a folder (for example) "in place"? And what good are they, if they don't produce an archive file? Gzip and xz do exactly what ZIP does, they create an archive file out of (a) source file(s).