Has Carol become unrealistically unhinged?

I think you're misreading what they've been doing with her character. She hasn't been becoming bipolar, she's been acting like a spy for her group since they first arrived at Alexandria.

Her transition from demure housewife to apocalyptic survivor who's prepared to do whatever it takes to survive (and general badass) has been going on for quite some time. Remember when people where getting sick at the prison? She did what she thought was necessary to protect the group and then played it cool afterwards. Rick, of all people, thought she went too far and had her leave the group.

And remember when everyone was trapped in Terminus? Carol didn't exactly ask politely for her friends to be released.

I'd suggest you rewatch the episode Indifference, way back at the start of season four (episode four): https://en.m.wikipedia.org/wiki/Indifference_(The_Walking_Dead)

The whole episode hinges on how much her character had already transitioned by that point.

With all that in mind, I don't think there's any question that she's only been pretending to be meek and innocent while in Alexandria. That incident with Sam wasn't Carol having a bipolar moment, she was lifting up the mask she'd been wearing so that she could make sure Sam didn't cause trouble for her.

Maybe you could argue that her character's changed to much over the series (though I'm inclined to think the apocalypse might have a profound effect on your character), but it certainly hasn't just started in Alexandria.

Also, while she, along with everyone else, misses the civilized world she used to know and the simple pleasure of making a good casserole, I wouldn't take that scene in the pantry to mean she was nostalgic for cooking for her husband. Ed was abusive, after all, and when she finds out what Pete's been up to she makes it clear to Rick that there's only one way to handle the situation, and making a casserole doesn't figure into her plan. (Well, not at first, anyway: https://m.youtube.com/watch?v=KQFFv2CFoTM .)

Personally, I felt like Carol was far from a blood-thirsty ninja when Alexandria was invaded. Yes, she saved the group, yet again, and killed without hesitation, but she looked like she was on the verge of tears when she had to euthanize the woman from the pantry. And when she takes one of the cigarettes at the end it really seems like the events have taken their toll on her. In that moment she doesn't seem the least bit blood thirsty. Rather, she understands what it takes to survive in the horrible world they're living in, and she's determined to do what it takes no matter how hard it is.

Really, despite all the unremorseful killing, there's a lot of subtle emotions running under the surface of Carol, and once you appreciate those her actions make a lot more sense. (Her character definitely wouldn't work without the amazing acting chops of Melissa McBride.)

I wouldn't say Carol has become unhinged, she's just very, very realistic about her situation and what it takes to survive. As shown by her having a bit of a breakdown at the end of "JSS", she still feels emotions over her actions, but understands that, in the moment, emotions and feeling sorry for the enemy can get you killed.

You have to remember that Carol, in a sense, was a survivor even before the show. Her husband was shown to be abusive, but despite everything she still cared for him and protected him when the world was falling into a walker apocalypse.

Carol is simply the kind of person who knows what's at stake and what it takes to stay alive when everything goes south. Rick disagrees with her methods when he learns she was killing people in the prison and teaching children how to use weapons, but when she unexpectedly rescues them from their captors in Terminus, after which Rick sees that Carol has a grasp on what's at stake, and permits her to travel with them without questioning her again.

Going to the season before that, in the infamous episode "The Grove", Carol does everything she can to make Lizzie and Mika understand the dangers they'll face every day. She cares about the girls and wants them to be safe, but when Lizzie kills Mika thinking she'll come back to life and play with her, she knows there's no way her or anyone else will be safe with Lizzie around and does what has to be done. Later, she leaves it up to Tyreese to kill her as well when she confesses she's the one who killed Karen and David back in the prison, trying to contain the illness, but Tyreese says he understands, and to me acknowledges that Carol is capable of seeing the problems and solutions that no one else sees, or is willing to see.

And really, that's been the main focus of the latest season so far: Rick's entire group seems to be unhinged, but at their core they simply know what needs to be done to stay safe, and eventually everyone else realizes that. They still care, they still want to protect people, but when people come after them they are no holds barred in keeping them and theirs safe from harm. Rick even does his level best to push the residents of Alexandria to face the walkers they encounter, even if it's with little success.

Carol and the others may be unhinged, but so is the rest of the world. The difference is Carol and the rest of the group still know who's good and who's bad, and act accordingly should the bad people reveal themselves.