You are indeed past all complex things. The N in the hdN:/netbsd boot
string is the number of the Apple_UNIX_SVR2 partition where your root
file system lives. The number typically is >= 9, e.g. on my G4 mac it
is 11. The output of the NetBSD pdisk command shows the number clearly
in the first column.
That is interesting info and it would indeed explain a lot.
As the filesystems where created with the installation CD (i.e. NetBSD
kernel), I guess they are based on the "disklabel" boundaries.
This would explain why openfirmware is able to access partition 2 (where
the start-of-partition information of the pdisk table and the disklabel
do match), but not partition 3.
I'll work on this tomorrow.