- You should pay strong attention to getting the syscall layer correct. If you implement the syscall layer well, it should be possible to use Mac OS X's userland on top of the lower level layers. That would enable you and others to compare the Mac OS X userland against the OSS userland components. That should make this significantly easier. It would also enable you to try a userland from Apple's Darwin images.
- You can use Gentoo Prefix to assist you in development. It is a userland package manager that lets you build a fairly large body of existing UNIX software for Mac OS X. What should be particularly interesting to you is that its dependencies are fairly light. Installing it should require only standard UNIX utilities, a toolchain, libc and the system headers. Once it is installed, it depends on only libc and the system headers. There might be a few other libraries that I missed, although they would be things like libm and libdl.
- If you are interested in seeing widespread adoption, it might be better to pick a less restrictive license. The WINE project uses the LGPL, which would probably work well for you. It is important to make a decision on this before you start receiving contributions, because it will be a pain to switch should you change your mind afterward. This is especially true with the GPLv3 because people tend to be very litigation-happy when things involve it.
- This is an awesome idea. Ignore people that say that this is a waste of time. Their opinions are not worth your attention.
I doubt that Darling would result in making Linux into a perfect replacement for Mac OS X, but I think Darling would make Mac OS X development on Linux easier. If Darling can obtain near perfect operation when paired with Mac OS X's actual userland, that would be a nice bonus.