Wow...sounds like you got a real live wire on your hands!
Let me ask, how much time, as a single mom now, are you getting with her on a day to day basis? Real quality time? And I mean like taking 20 minutes out of your day to sit down and play a board game with her, paint her nails maybe, even watch a cartoon together with you sitting next to her, cook dinner together maybe. I know its easier said than done, I'm a single mom too and time is certainly fleeting, but maybe she's acting out because it gets under your skin and it feels like to her its the only time you pay attention to her.
I would definitely consider maybe a little counseling. Talk to the school counselor, see if they can point you in the right direction, or if your daughter will open up to them.
Positive reinforcement is good, I believe that, but I think taking all her toys away was a very good move too. She has to understand that behavior like that, for whatever reason, cannot and will not be tolerated and has consequences. If my son has really gone and pushed me over the edge, TV, video games, and occasionally his treasured bedtime story, are gone until he can behave for a day or two. Maybe for your daughter, she should spend a few nights grounded, able to do nothing but sit in her room with no toys or TV, and a 7:30pm bedtime to get your point across. Make sure she knows you love her, but you won't put up with this. My son is 6 too, he would totally get the picture and be miserable like that for a couple nights, it'd be Niagara Falls in our house for part of it I'm sure, but I guarantee you he'd shape up. If your current custody agreement conflicts with that, I think you really need to consider re evaluating it. If your ex has her every night after school, your daughter knows full well there are no consequences for her acting like that.
I think you have a tough situation on your hands, and even though it will ultimately pass, its going to be hard for you to deal with until then. Try a few things, see what works, every parent and child is different. I myself am more of an authoritarian type parent and kind of rule with an iron fist, so I don't think punishment is ever a bad thing, but I'm sure other parents would freak out about it.
PS...I have found in my short 6 years as a parent, a bar of soap works wonders to get a kid to shape up. Just an extra thought.