What undocumented APIs / interfaces are not understood? Would seeing Microsoft source help?
The best solution would be if the Windows API were fully documented, so Wine could be a perfect “clean-room” implementation. Seeing the source code might make it harder to prove that no copyright violations have taken place. That said, the documentation is often bad, nonexistent, and even misleading where it exists, so a fair amount of reverse engineering has been necessary, particularly in the shell (Explorer) interface. The biggest problem facing Wine though is simply lack of manpower. At one point, over 5000 people were working on Windows 2000. While Wine doesn’t need to replicate all of Windows (we only cover the parts needed to make Windows programs work), that’s still nearly 8 times more people working simply on one release than have ever worked on Wine, in the history of the project.