From what I know of, there isn't a way to do this, as I have barely kept up with anything other then LG devices (I now have a LG G5 and it isn't rooted cause I just don't see a need for it) so I can't really help.
My first guess would be that it needs to find a permanent root before you can flash a recovery, no matter how much in adb you use, root will always be needed. I would look for a similar phone then attempt the root from that on your, like the L41C was to the L70.