If you refereed to the NT kernel to be a monolith, then it actually is a hybrid. Other than kernel-integrated UI, its NT core is micro-kernel by design.
The UI kernel integration intrigues me. Apparently it was done for performance reasons. If it improves performance, then that would be a legitimate reason to do it.
I guess the project is aiming to hit some critical mass where it attracts more developers and becomes self-sustaining. But does that point change with each release of Windows?
Second, yes again - UI was integrated due to performance improvement. Earlier versions of NT had microkernel architecture.
Third - well, this is a philosophical question. There are a lot of different people involved in ReactOS and I guess each of them have slightly different reasons for their involvement. "Critical mass" might be unavoidable though, considering the offering. For now Windows 2003 is targeted due to its simpler kernel architecture (NT 5.2) compared to later versions. After that will be achieved, well... we'll see.
Care to elaborate on the kernel-integrated UI. That seems like a weird place to break from a micro-kernel design.
"The emulation subsystem which implements the Windows personality is called the Client/Server Runtime Subsystem (csrss.exe). On versions of NT prior to 4.0, this subsystem process also contained the window manager, graphics device interface and graphics device drivers. For performance reasons, however, in version 4.0 and later, these modules (which are often implemented in user mode even on monolithic systems, especially those designed without internal graphics support) run as a kernel-mode subsystem[1]."
But the fatal flaw could be its monolithic nature. Linux is just a kernel. Unix, by nature, is more easily broken down into a loose collection of components that can more or less stand on their own. NT was created as a monolith by one company with tremendous resources, and it doesn't easily break down into smaller pieces. It either all works or it all doesn't. So this project seems to have a much bigger scope than Linux; it's more like Linux+GNU+X11+KDE+Wine+... all rolled into one. Unless they've figured out how to drop in chunks of NT itself and have it work. So I wish it luck, but it sounds daunting...