Configuring USB tethering manually with root
A buddy let me know that there is more than one sysctl setting for forwarding (a global one and one per interface, then more for IPv6). 

On my phone, the relevant settings are:

vars=(
    net.ipv4.conf.all.accept_redirects=0
    net.ipv4.conf.all.forwarding=1
    net.ipv4.conf.default.forwarding=1
    net.ipv4.conf.ip6tnl0.forwarding=1
    net.ipv4.conf.lo.forwarding=1
    net.ipv4.conf.p2p0.forwarding=1
    net.ipv4.conf.rmnet0.forwarding=1
    net.ipv4.conf.rmnet1.forwarding=1
    net.ipv4.conf.rmnet2.forwarding=1
    net.ipv4.conf.rmnet3.forwarding=1
    net.ipv4.conf.rmnet4.forwarding=1
    net.ipv4.conf.rndis0.forwarding=1
    net.ipv4.conf.sit0.forwarding=1
    net.ipv4.conf.wlan0.forwarding=1
    net.ipv4.ip_forward=1
    net.ipv6.conf.all.forwarding=2
    net.ipv6.conf.all.proxy_ndp=2
    net.ipv6.conf.default.forwarding=2
    net.ipv6.conf.ip6tnl0.forwarding=2
    net.ipv6.conf.lo.forwarding=2
    net.ipv6.conf.p2p0.forwarding=2
    net.ipv6.conf.rmnet0.forwarding=2
    net.ipv6.conf.rmnet1.forwarding=2
    net.ipv6.conf.rmnet2.forwarding=2
    net.ipv6.conf.rmnet3.forwarding=2
    net.ipv6.conf.rmnet4.forwarding=2
    net.ipv6.conf.rndis0.forwarding=2
    net.ipv6.conf.sit0.forwarding=2
    net.ipv6.conf.wlan0.forwarding=2
) ; sysctl "${vars[@]}"


With these and the other ones, forwarding and tethering works!