>   a) is the iBook supported?  [maybe INSTALL needs mention]

 It is NOT supported on 1.5BETA I am afraid, but with minor two 
patches, iBook runs NetBSD. I am running the machine with them.
port-macppc guys are still working on that patches and it may not
be official yet, but I was eager to get them. They are at least on
my ftp site. 

ftp://ftp.ki.nu/pub/NetBSD/port-macppc/dev-adb.c-patch
ftp://ftp.ki.nu/pub/NetBSD/port-macppc/wdc-problem-fixed-by-cpu.c

With that adb patch, kernel stays some 20/30 seconds
to probe ADB keybord. Be patient.

>   b) is NetBSD easy to install on iBook

It is hard to tell to say if the work is easy....

(1) sysinst is not ready yet, so you need manual install.
(2) at least you need another machine to compile the kernel 
   with above patches. 

I have no idea on the rest of the questions, sorry.

You may need be careful on window systems.
I have tried two X Window systems, NetBSD distribution and
XF86. 
  NetBSD distribution seems to me unstable, while XF86 4.0.1 runs
OK. But on my machine, I get only 640x350 pixels yet.

Good luck,
---
(Makoto Fujiwara)