Alice
after analysing the scenes you recommended and things you said, here are my thoughts.
on a little doll (used for stop motion scenes) that is used when a terminator goes in the corridor (right before Sarah and Kyle shut the door) the parts don't move. but the torso easily bends/swings forward and backward. same happens later, when the doll is filmed from the back.
then there is that scene that you mention - where obviously hollow parts move independently, and they move not by terminator's will but by a pipe bomb.
i think it's well seen here that they are not supposed to bend. plus it's very well seen in movement that they are hollow.
that gave me an idea... what about those things aren't spine but are just
armor?
then there are these shots:
1) a part of spine with a hole in it lies near Sarah's hand.
2) spine stripped of plates. control and power cables only! of course there can be something in the middle, but anyway those spine plates are not part of movement mechanims, i'm sure of it now.
neck is debatable, but that part should definitely have some inner core/rod, or terminator's head would depend only on two thin hydraulics that actually cannot even hold the head straight, if there is no third rod.
i think it's best to assume that such rod is also present inside the torso's spine - for the same reason. a terminator wouldn' be able to properly operate his body on only two hydraulics (like Cain in Robocop 2, who had 4 of them and no spine column at all, only control cables).
so a properly supporting but at the same time bending system inside spine armor plates is what i think now.
so... an original terminator has a fortified spine with really thick plates covering it, while Gynysys model has it all outside - and thus looking a lot thinner.
what its author thought to be an improvement turns out to be a worse model (just like you said) - it has no plating over mechanics.
i'm so sad that new terminator's height is so out of scale. this new detail goes perfectly with all other exposed mechanics into my now abandoned idea of using it as a more fragile, less dangerous T-700 series from Reese thoughts in the novel.
also now Cameron's sketch is more clear: plates, not actual mechanics.
i don't know how i forgot about those plates with holes in them (the one near Sarah's hand) after using them in exactly that manner on my blown-up T-800.
p.s.