L4linux is not a microkernel. L4Linux is a user space server, provides Linux compatible ABI. L4 kernel helps to make it.
How the memory protection is addressed by L4Linux if it is a L4 userspace process?
The L4Linux kernel and the Linux user tasks run in separate L4 address spaces each. Does this answer your question?
Yes, I think so. Therefore, the L4Linux syscalls are implemented as IPC calls too?