yes the high back should be paralle, but i do not keep mine paralle. i just got used to it how it is. i believe that it helps with heel side turning but i have no idea.
how this ties in with forward lean idk. my bindings are burton cartels and they come with zero lean. just the way i like it.
maybe this will help with that.
What is Forward Lean on Snowboard Bindings? - The House Boardshop