First yes, monolithic kernels aren't any harder than microkernels, they are actually easier. That was the main reason for Linus' choice when redesigning the Minix microkernel into monolithic Linux.
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.
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.