Flow requires difficulty that correctly matches skill. If it's too easy, it isn't a state of flow. Rather, it's just coasting on autopilot. Flow usually equates to deliberate practice.
I don't believe this is true at all. If it's too easy, sure, that's not flow. But same is true if it is too hard. Deliberate practice intentionally and introspectively focuses on changing your approach. If done correctly, I think this will easily break flow.
Deliberate practice is never relaxed - but flow can be.
To reference the OP, there was discussion of intentionally focusing on and making the difficult parts of a piece harder as a form of practice. That doesn't bring flow, but allows the flow state later when you play the piece "for real". I think this generalizes well.
No, the kind of state of flow you're mentioning lacks the necessary consciousness for it to be deliberate practice. The emphasis on deliberate: you can either focus on executing over a period of time (flow) or focus on being mindful about how you're executing things (deliberate practice). Sure, there's an overlap, but I think the concerns you'd have to deal with each are completely separate from each other.
