It sounds as if you went from dad to step dad very quickly just from the timeline, unless I'm reading that incorrectly. That's hard on a kid and they don't adjust as fast as parents think they do (did you/she/family get any counseling during this time?). They will say take it super slow and don't move in together, so that could be what caused the issues to begin with.
That being said, attention - negative attention included, is attention - and to me this sounds like a ploy for attention. A lot of women, including a friend of mine, unfortunately make time for their new men - over their children, when they enter into new relationships. I find this really hard to watch. So if you went into a new relationship with step dad, and were 'courting' that's going to be hard on a child. You're definitely giving him more attention to new man than you did her father. You're also giving more attention to yourself than you did when you were with dad too. Not that that's necessarily a bad thing, but it's a big switch for a kid.
So jealousy is not really jealousy - as in, uncalled for. It's that kids feel they should still rank as important as they once did, and don't get why they don't. She obviously doesn't understand why things changed, and then tries to get attention, then worries (anxious) that you'll be upset with her ....
It all sounds really anxious and upset and almost hysterical to me.
I wouldn't engage at all - in trying to make your case. She needs reassurance. I think counseling is the way to go.
You go, she go, and then go together to joint sessions.
Where' dad in all of this?
Sounds like she needs stability.
Toddler tantrums are not at all the same as teenager/pre-teen or hormonal angst. Toddler tantrums are generally from being overtired or parents not reading cues of their kids. I know, because my first had them until I became proactive. Some kiddos have them just because that's their temperament and there's not much a parent can do other than to try to head them off .. but to compare it to your daughter here .. I think your daughter is trying to tell you she's really unhappy.
My preteens and teens could be difficult/challenging, but this seems pretty extreme. I just wouldn't engage, because you're reacting. I took mom moments. You don't have to engage - period. I don't try to reason with unreasonable people - kids included. You don't have to. It's pointless. You're giving validity to what she'd doing. A time out could be she goes to her room to cool off, lie down, relax, chill out - until she can be calm and respectful. I never 'discussed' stuff with my one kid who needed time to veg at end of day.
Good luck. Sometimes preteens really can benefit from a counselor just to get stuff out - even if they're not that troubled, but feeling emotional and need someone not related to talk to.