I'm also someone who hated Natalie at the start, and cannot believe I am rooting for her now. I think I understand her thinking in wanting to keep Keith and dump Alec, and not even consider voting for Jaclyn this week. As many of you have pointed out already, Keith is someone who could be useful in keeping Jon from winning immunity. But also, I think Natalie has more confidence in being able to keep Keith loyal to her than she would with Alec. Especially after seeing Baylor softening up to Alec shmoozing her earlier that day! It's true that Baylor ultimately didn't buy what Alec was selling, but given a few more days out there together, and Baylor's possibly being an apple that didn't fall far from the Missy Tree, who knows what headway Alec might have made there. Alec isn't getting any jury votes, but he might have been able to sway Baylor (and Missy) to turn against Natalie and vote her out, after they got rid of J/J. And I think that not voting out Jaclyn can be explained fairly easily, too. It's part of keeping J/J comfortable until she can get rid of Jon. I'm sure that if Jaclyn had been voted out, Jon would have become angry, and felt betrayed, and started to play much more aggressively — having belatedly realized that he is not in control of the game. Giving him a reason to make more of an effort just seems to guarantee that he'd go on an immunity run, and end up sitting at FTC. I don't think Natalie flipping her vote to Alec is that much of a gameplay faux pas as some of you think. Besides the "oops, I thought I was supposed to vote for Alec" play, she could also confide in Jon and Jaclyn that she feared Alec was going to sway their loyal alliance-mate, Baylor, with his wily charms. Jaclyn would certainly believe that, and Jon is so easily led that I don't see it problematic at all for Natalie to convince him of it, too. And Missy and Baylor wouldn't need any handling with regard to this because they believe (correctly, I think) that they are in an ultimate F3 with Natalie, anyway, so as long as her move didn't jeopardize them, I don't think they care one way or the other. Especially when Natalie explains to them how Keith is more valuable at keeping Jon from winning back-to-back immunities. I still think Natalie could pull out a win based on this strategy. Except for blood loyalties (i.e. loved ones on the jury), it's likely that any non-bitter jury members would feel compelled to reward her with the win for her gameplay, considering she's the only one left who has any gameplay.
Edited to add: wings707 mentioned something I forgot to post, but agree with: if they'd voted out Jaclyn and Jon aligned with Keith and Alec, it's not good for anyone because it would lead to PROD or fire building. No one wants to go out that way!