Hi there, I am not bipolar but I did take Seroquel at one time for insomnia, not the same, but thought I would reach out. First of all, going cold turkey off methadone is some serious shit and I really, REALLY think you should think about tapering instead, especially since you could exacerbate your existing mental health condition. Not trying to scare you, but really - you don't want to mess around with that kind of cold turkey. It makes heroin withdrawal look like a walk in the park.
Second, Seroquel is going to make the jimmy legs/RLS thing 1000 times worse when you are kicking. It's been discussed a lot on this board - I know that info is sort of useless to you and probably sucks cause I know you have to take the meds for your bipolar condition. A few things I have tried - tight knee high socks. I know that sounds silly, but it can help. Heat/Ice. And I have taken those Hylands Homeopathic Quinine tablets. Not sure if it was placebo or what but they seemed to help.
Lastly - can you talk to your psych doctor and see if he can adjust your meds while you kick the 'done? Because it's not a quick kick - methadone has a crazy long half life.
Good luck, man. Been there, done that. It gets better.