Generally, it's easy. My state variable represents a single concept in my problem domain, so there's usually only a single source of truth that would ever update it.
> What if two or more bits of logic try to set this variable at the same time
Generally, this will only happen if the variable is actually a derived property, rather than a primary one. I handle this by making my primary properties observables, and my derived properties read-only computeds. Taking this approach, I don't think I've ever had the quandary you describe, and I've built everything from very thick SPAs to very thin and nimble content sites.
> How will you control that `myvar` can only be set to particular values dependent on other state?
It seems to me a matter of simple discipline.
> How will a QA engineer demonstrate to you that it was this state change which broke your application?
She might write a failing test case, or use the debugging tools I've added to the console. She certainly wouldn't mess about with something as proprietary as Redux devtools, because at that point I'll be unable to move to better JS technologies without getting cross-team buy in. At most companies, that basically means it just won't happen.