For the fingers and jaws you can do some puppeteering to make extensions of your motions within the suit with pulley systems. Here's a good sample articulated hand tutorial.
Articulated Skeleton Hand: Back in 2014 I created the Ultimate Grim Reaper Project: Ultimate Grim Reaper Costume - Instructable. One of the hardest parts of that costume was creating hands that look like bones but can also stand up to a lot of use. I had to settle for smal…
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Using elastic material to hold the joints in their natural position and a cable to pull them along a path to close the hand into the fist or pull the jaws open is probably your best bet. The hardest part to that would be keeping scale of the Sangheli body to be able to hide your hand and the pulley mechanism in the arm and a chin based lever and helmet inside the neck/head.