in that example, unlikely to be any parallel paths.Come on then explain.....your cooker cct has no cooker attached it is not connected to anything from the DB all the way to the outlet....where are the parallel paths?
but if you have a spur feeding a boiler, you then have a parallel path through gas & water pipework back to MET. which can then end up with Zs lower than R1R2.
same would apply to an instant water heater connected to a ring main....
or an electric shower
now, do you still want to be a **** and say it can never happen?