I can only answer a few:
- can you use an earmv6hf build to install on RPI2/3? If so, is there an included kernel7 and does that use earmv6h or earmv7hf instruction set?
earmv6hf will work fine on any Pi. You can also use an earmv7hf kernel with an earmv6hf userland.
The v6 images come with kernel7.img, compiled for v6, so should work on any Pi.
- if you build with eb, does it work?
No. Banana Pis and Cubieboards, for example, can run as big endian, though.
- do the earm/v4/v5 builds do anyting useful? (with the understanding that surely one should use v6 on a RPI1)
There are still a number of earmv5 devices out there like Seagate Dockstars and PogoPlugs.
- does earmv7 run faster than earmv6 on a RPI2/3? How much?
Not measurably, in my experience. Haven't looked in to that in a couple of years, though.
- I think there is no documentation or script to update firmware on a system once installed, other than a cowboy copy of most of /boot from an unpacked rpi.img.gz or armv7.img.gz. And that the firmware is not in the installed filesystem anywhere. Correct?
This would be a nice thing to document. It might also be nice to have an official NetBSD place from which to fetch this :)
John Klos