Would Chigurh have let Lewellyn live?

This is answered in the movie. He would've killed him.

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Maybe if Chigurh had bumped into Moss as soon as he was hired to find the money, he would have spared him. He let the trailer park owner live after refusing his requests for information. He lets the motel owner live after letting him see his face and asking for a map of the rooms. He lets the Accountant live after seeing him commit a murder.

If he'd asked Moss for the money as soon as he set out to find it, and Moss handed it right over, than perhaps Chigurh would've spared him. If you cooperate with Chigurh or not impede his progress at least too much, then Chigurh won't kill you.

Wells points out that "He'd still kill you just for inconveniencing him." Inconveniencing is the key word. By the time Moss meets Wells, Moss has already outsmarted and fled from Chigurh at twice, even going so far as to wound Chigurh with a shotgun which forced Chigurh to (painfully) repair himself.

Moss had gone too far to simply hand over the money. In the beginning if he'd simply handed it over then Chigurh might not have killed him, but as soon as he decided to run and actively fight Chigurh then Chigurh was going to kill Moss no matter what. Chigurh even killed Moss' wife because he gave Moss his word that unless Moss came forward he would kill her, and Moss didn't.

Moss inconvenienced him by running, so Chigurh was going to have to kill him. But Moss also physically wounded Chigurh, so Chigurh was also going to have to kill him for that too. Perhaps that was also a factor in killing Moss' wife, Chigurh finally getting revenge on the man who wounded him even though the Mexicans got to Moss first.

Can't be too careful with all those weirdos running around.

It's very possible Moss would get a coin toss.

If Fate has ordained that Moss shall certainly die at the hand of Chigurh, then Fate is playing pretty serious games with Chigurh in letting his prime tool be grievously wounded and letting Moss escape. That could plant doubt in Chigurh's mind as to what Fate has in store for Moss.

Chigurh tells Moss, "I'm won't tell you, "you can save yourself," because you can't." That's a very different statement from, "no matter what you do, I'm going to kill you. The difference is that if you bring me the money, I kill only you. If you don't bring me the money, I kill you AND your wife." Yes, we read that implication into the exchange, but it's not actually said. Chigurh is carefully and deliberately ambiguous.

Throughout the film Chigurh sees himself as the tool of Fate, meting out whatever result Fate implants to his mind. In this sense, Chigurh is indeed crazy as Wells accuses him of being. The rest of his actions are ordered carefully around his delusion as Fate's hand. In that sense, he's not insane; rather, he's acting reasonably within that structure.

There is absolutely no doubt that Chigurh will ultimately consult Fate as to what is to be done with Moss. We can certainly imagine Fate (Chigurh's unhinged mind) responding, "he must die: be done with him." But there is also the possibility that had Moss done as Chigurh/Fate directed and brought him the money, that the coin would have been consulted, just as it was for Moss's wife and for the storekeeper he spared. Heads or tails, Fate orders Chigurh to spare or to kill. There's not anything Moss himself can do to either save or doom himself. The only thing that matters is Fate dictating to Chigurh and Chigurh acting accordingly. He does have, after all, those principles to uphold.

He let the trailer park owner live after refusing his requests for information.

No, and he makes this abundantly clear in the phone call.

The latter.