Okay, I think I got everything!
I updated
the KOReader build, please check that I didn't miss anything

.
See PR
#335 &
#1620.
Note that wpa_supplicant & the dhcp client still won't get killed on exit if started without nickel running. Since that's not something I'm doing, I have no idea if that's a desirable behavior or not (see platform/kobo/nickel.sh for how it's done with nickel running).