Yes, I agree entirely, I think it is a combination of PBS install on top of debian via APT / and some unwanted interaction that happens when ifupdown2 gets added into mix. I'm not even convinced that ifupdown2 is 'really mandatory' so much as 'nicer in theory'. ie, my semi-insane manual tidy-up steps posted above, appear to leave us with a functional PBS server / with no ifupdown2 present / with just vanilla ifupdown present.
Lots of fun.
Lots of fun.