Are atheists and theists capable of switching roles and sincerely arguing for or against God?
A definite “no”, for this atheist. I can debate pretty well, and one of the fundamentals of debate is that truth is not a requirement for effective debating… but debate is just a tool. One wielded by a human being, who — bear with me, I know this is quite old-fashioned — are supposed to have a notion of morality, and try to cleave to it. To put it in a snarky fashion, “to use their powers for good, and never for evil.”
So while I could theoretically argue for a theistic position, it’s not something I would ever do because I see supporting theism — and I’m talking about Abrahamic religions as they are today — as immoral.
I could probably take a fairly decent shot at arguing for deism, or perhaps Buddhism (which many consider a philosophy rather than a religion); taking the position that there may be an impersonal force for good in the universe, or that there’s a way to reduce suffering, is not something that I would find morally objectionable. But command moralities, with “gods” for whom you’re supposed to feel abject terror and love at the same time, and that behave like toxic shit weasels without the slightest shade of a rational excuse? No can do.