Ralph Ronnquist:
...
> Afaict, mdev, udev and vdev are all "just" different rule interpreters
> by which to define what to do upon discovery of hardware elements. But
> none of them contain the actual rules. Therefore e.g. "mdev -s"
> doesn't do anything; it would if I had a good /etc/mdev.conf and
> that's where it stops for me.
...

If you look at:
https://git.busybox.net/busybox/tree/examples/mdev_fat.conf

you see that mdev.conf is for knowing owner:group and chmod of the
device file to create, example:

ttyS[0-9]*      root:uucp 660

You can also run a program when creating a device

sd[a-z].*       root:disk 660 */lib/mdev/usbdisk_link

but thoose programs are not provided by the busybox git repo.

///

I'll see if I can find some informations re. you other points.

Regards,
/Karl Hammar