Re: Why the lack of personal protection?
I'd think a device designed to contain air would be very good at resisting gradual, evenly spread pressure, even fairly high pressure. Not so good at resisting sharp piercing pressure...
Whether the belts do zip against phasers in "Beyond" is somewhat debatable. People being shot at are all in considerable pain, it seems - which is the aim of the creature controlling the phasers (or whatever those beams are). But Kirk resists the beams just as well after removing the belt, if not better.
..Yes, the animation is again wildly inconsistent on whether Kirk has his belt on or not. But he makes a great show of removing it and placing it on the control console, in an apparent gesture of surrender, so we are supposed to believe he indeed ceases to wear a belt at that point. And the yellow glow around him at beam impacts is probably due to the beam, rather than the belt...
He's put one hand on the belt that he left on the console to short it out, so I'd say the yellow glow is from the belt.