Their dreamless slumber was ended by Ben shaking them awake, “Get up, we’re landing in a few hours.”
It was dark outside of the jet and Cedar could see stars shining over a desert that they assumed was Pakistan.
“How long was I out?” Cedar asked as they got up and followed Ben out of the bedroom.
“Little under eleven hours,” Ben answered. “Not that I blame you.”

First crit back and it's a petty, detail oriented one. Classic AKA-M80, amirite? Anyways.
The flight time between New York and Pakistan (assuming Cedar is correct in their guess) is ~18 hours. Now, that's commercial, and private planes typically fly faster, but it's not enough to shave off that many hours. And the sleep seems to hit Cedar almost immediately, so they would have been asleep for roughly 18 hours, not 11. Again, presuming Cedar is right.
I was gonna put some stuff here about Pakistan to Everest, how Lukla Airport, the closest one to Everest, is actually around two hours making that part right, but it's something like a 13 day trek to Everest, but then I read ahead a bit. Nevermind.

They gulped nearly half of the bottle down, “You get any sleep?”

Not necessary, but I think the action would transfer to the statement with something in between the gulping and speaking. Something like "… down, taking a deep breath before returning to the conversation, …" or whatever, just an idea, but it's fine without it.

Maybe that was it, Cedar was a stupid adrenaline junkie that’d be perfect for doing dirty work.

Interesting. Perhaps a man vs. self conflict for later? Just saying.

Now, he had dawned a black turtle neck and cargo pants.

"dawned" should be "donned"

Ben was silent. He knew that Cedar could have just been saying all that, but the words rang true to him. He stared at the engine as he remembered his vowel to protect the Wanderers’ Library from all threats. He focused all of his energy into the engine. A blood vessel burst in his eye but he ignored it, not moving his eyes from the engine until he felt the pulse.

Personal opinion incoming: I think you can stay on this more. This is a good character moment for Ben, let the text sit on his dedication and the strain for a moment longer. Really show that this is hard for him, but that he's too dedicated to The Hand and the Library to care, pushing through the strain. Get spacey, get into short paragraphs.

The explosion was so loud that it cracked the windows. Fire and black smoke erupted from the engine.

I think you can combine these two ala: "The explosion was so loud that it cracked the windows, fire and black smoke erupting from the engine."

As he death-stared Cedar, Johnson struggled to wrap his fingers around the handle of his pistol.

I'd swap these. "Johnson struggled to wrap his fingers around the handle of his pistol, death-staring Cedar."

The plane was hurtling towards a cliff and fast. He hit a release switch and a massive parachute shot out of the back of the plane, slowing its descent.

I was gonna crit then, then looked it up and apparently this is a real thing. Learn something new everyday.

Johnson fell forward onto Beau and Cedar fell backward into the cockpit.

You need to separate these two things in some way, whether it be a comma or a period. As is it's a bit confusing to read.

but they just couldn’t get enough leverage.

I'd nix "just"

Johnson slowly rose to his feet. Seeing, the struggle in the cockpit

I'd combine these two ala: "Johnson slowly rose to his feet and, seeing the struggle in the cockpit…" (And nix the first comma like I did in the example.)

“You had that the whole time?” Cedar asked, nodding at the knife.

He used it to cut off their cuffs, why is he confused?

Cedar pulled a bandage and rubbing alcohol from the first aid kit.

Cedar's torrid affair with rubbing alcohol and wounds continues to be my favorite subplot.

Cedar winced as they poured the rubbing alcohol over their wound. Tossing the bottle to the side, they began wrapping the bandage around their thigh, “Ok, good, I think it missed the bone.” They pulled up their jeans and stood up on the wall.

More of a personal idea for flair, but did the bullet go through cleanly? Is it stuck in the flesh? Since the bullet missed the bone, then it comes down to your interpretation of the GOC as there are bullets meant to not pass through the body, even if it misses bone.

“I made it!” Cedar called. “I see some light deeper in. I’m going to keep moving.”
“Understood, Bardot,” Beau yelled back. “We’re right behind you.”

I call shenanigans on being able to communicate through voice alone for thousands of feet. But then again, this series hasn't ever been too concerned with reality to do with this info what you will.

they identified which direction the chemical smell was coming from.

If the smell was pervasive enough to be smelt while dropping into the cavern, it'd be extremely difficult to pinpoint its location that further in. But, again, something something suspension of disbelief or whatever.

shoved a clump of the stuff into his gums

A petty, petty detail, but as a homegrown Southern boy, I feel obliged to mention that you pack it between a lip and gum, not, somehow, into the gum.

He didn’t have time to get a word out, air was quickly escaping him.

Again, petty detail, but if he can't take in air because of the choke then he can't push air out either. The suffocation comes from the body turning it all into CO2 and it not leaving to be replaced with regular ole O2.

and bare scan it before

I'm presuming you meant "barely"

Cedar picked up a piece of parchment and tried their best to flatten it out.

Is this the same paper the GOC grunt crumbled? Then the "a" needs to be "the." If not, why are they trying to flatten it out? Is it folded? Wrinkled?

The was a dead flower in a jar

Presumably, "The" —> "There"

They unscrewed a jar with a, CRACK A lion’s rotting fetus.

Formatting's a bit screwy

He spat out a glob of tobacco before speaking.

Is he spitting out the chewing tobacco or just some delectable, juicy, deep brown tobacco-laden saliva? (Yuck.)

take his rifle off safety.

Eh, wording here's a bit wonky. It'd sound better if the solider is "struggling to turn the safety on his rifle off," if that makes sense.

“Watch the fire!” A soldier yelled. “We want them alive, don’t we, captain!”
“I don’t care if they’re alive or dead!” The captain called back. “Just get that damned cat!”

Shoutout to the GOC for respecting Cedar's pronouns.

The guns suddenly stopped, and Cedar heard the dry sound of a shotgun cocking. The sound was followed by swift footsteps and then a sudden shot, blowing a hole through a shelf. The shotgun cocked again.

I'd nix "The sound was" and replace it with a comma, lets the scene flow quicker.

Also, with the number of booms before a reload and the fact that the chamber started dry, seems homies is weilding a Mossberg. Hella.

The captain wasn’t given a chance to scream as the figure seized his other arm and tore him in half in the blink of an eye.

Yet again, my petty lack of suspension of disbelief rises. One of the arms would be torn off LONG before he got ripped in half.

Tall and horned, she was dressed in long, red robes typical of a Daevite priestess.

Hot. ^{(it is 12:02 am and I am tired please forgive me)}

You stupid fuck- *cough* fucking asshole!”

Just a note to fix the formatting

the writing corpse of Marw

Presumably, "writing" —> "writhing"

All in all, I really enjoyed this. My biggest criticism is the sentence structure for most of it. I pointed it out in a few places, but overall it's very start-stop-start. I think you'd benefit from going through and trying to give it some good flow, combines sentences, less "then"s and so on and so forth.

Sorry for all the nitpicky detail crits. Though that's a bit hollow, 'cause I'm not actually all that sorry. :)

Keep up the good work and good luck!