1 - Like the other Escapist said. He would die for sure after some days. Except of course if he drink his own salty tears. Ho ho ho.
2 - I think Nathan said this lie about Ava not wanting to escape to make the experiment more successful. If Caleb knew that Nathan at some point wanted to escape, there would be a chance he wouldn't wanted to risk helping Ava.
Also Do you remember this dialog:
Nathan: You know this guy, right? - Jackson Pollock. - Jackson Pollock. That's right. The drip painter. Okay. He let his mind go blank, and his hand go where it wanted. Not deliberate, not random. Some place in between. They called it automatic art. Let's make this like Star Trek, okay? Engage intellect.
Caleb: Excuse me?
Nathan: I'm Kirk. Your head's the warp drive. Engage intellect. What if Pollock had reversed the challenge. What if instead of making art without thinking, he said, "You know what? I can't paint anything, unless I know exactly why I'm doing it." What would have happened?
Caleb: He never would have made a single mark.
Nathan: Yes! You see, there's my guy, there's my buddy, who thinks before he opens his mouth. He never would have made a single mark.
Nathan: The challenge is not to act automatically. It's to find an action that is not automatic. From painting, to breathing, to talking, to fucking. To falling in love...
Nathan: And for the record, Ava's not pretending to like you. And her flirting isn't an algorithm to fake you out. You're the first man she's met that isn't me. And I'm like her dad, right? Can you blame her for getting a crush on you?
This a VERY important part of the movie:
Why Ava doing all this? She/it wasn't doing this because she/it fell in love with Caleb in the first place cause it was a "moment". an automatic thing to do so she can escape.
She is a Machine after all to her core, literally. She NEED to caculate to have a plan so she/it can escape.
But then you ask "But isn't this the opposite of automatisation? To have a REASON doing all this?"
No, because then they had this discussion:
Caleb: Did you program her to flirt with me?
Nathan: If I did, would that be cheating?
Caleb: Wouldn't it?
Nathan: Caleb, what's your type?
Caleb: Of girl?
Nathan: No, salad dressing. Yeah, of girl; what's your type of girl? You know what, don't even answer that. Let's say its black chicks. Okay, that's your thing. For the sake of argument, that's your thing, okay? Why is that your thing? Because you did a detailed analysis of all racial types and you cross-referenced that analysis with a points-based system? No! You're just attracted to black chicks. A consequence of accumulated external stimuli that you probably didn't even register as they registered with you.
Caleb: Did you program her to like me, or not?
Nathan: I programmed her to be heterosexual, just like you were programmed to be heterosexual.
Caleb: Nobody programmed me to be straight.
Nathan: You decided to be straight? Please! Of course you were programmed, by nature or nurture or both and to be honest, Caleb, you're starting to annoy me now because this is your insecurity talking, this is not your intellect.
In other words, sometimes, you DON'T have a choice at all. It just happen. She/it just happened she/it need to escape. Being just a random moment base of many factors, she only want now to escape as fast as possible.
Even if we say she have plans to do all these things in the whole movie, we don't know if she/it automatically just needed to escape. Not because she have a reason. Just because.
Very good movie btw.