What's Happening With Nebula In Avengers: Endgame?

If Nebula is on board the ship with Tony Stark, then I find myself wondering why they're about to die. The Guardians' ship is a place where six people live, and yet two people have run through all the food, water, fuel, and oxygen on board? If Tony were left alone on the ship that would be one thing. He'd probably be smart enough to get it in the air, but he'd have no idea how to navigate space in a way to get home. He could easily run out of fuel, and then food, trying to find the way home. However, if Nebula were with him she could almost certainly pilot the ship to Earth.