Interface configuration wonkiness when using extroot

Yes but if i understood it correctly, the extroot boot process is just mount the target block device to /overlay and bind mount some directories (e.g. /sys, /proc ..) to the corresponding location then chroot into it. This process is done before uci configure the whole system using the configuration. So I don't think the original configuration will affect the system

Yes, I concur that the original should not be parsed... however, I also think that it is worth trying it.