I find it can be helpful at times to replace words we've become accustomed to with alternate terms.
Hermes, as far as I can see, you confine the word "mindfulness" to one particular form. Scientists have recently been attempting to describe several different kinds, some of which involve the kind of open awareness you describe, yet others involve focused attention. And some even involve the kind of active thinking (related to the prefrontal cortex) you mention as associated with flow.
I've often found with my sleep students that talking about effortless and awareness and "attention" can be more helpful than the single word mindfulness.
Dr. Les Fehmi, over more than 50 years, taught four major forms of attention: wide and narrow detached attention, and wide and narrow immersed attention (with ""detached" and "immersed" being pretty much experienced as you might expect - if this is not clear please ask).
He then would teach "Open Focus," which involves a flexible attention stance that would allow attention to spontaneously shift as circumstances call for it. This very much lessens the ego involvement that interferes with flow, and I would say, characterizes many different flow states.
Although immersed attention resembles the "merging of action and awareness," Open Focus encompasses - ultimately - the full range of possibilities of attention.
So it would include what you experience in practicing mindfulness, in spontaneous flow experiences, in Zen session, and, well, every other human experience.
Open Flow also involves what Loch Kelly (and now, Internal Family Systems) identifies as "effortless awareness." He provides over 50 practices in his books for recognizing a calm, peaceful background awareness which is always present, does not involve strained effort to become aware of, and which leads to great improvement in the ability to "navigate" attention as occurs with Open Focus.
As a neuroscientist, you may be interested in the research of Zoran Josipovic at NY University.
Excellent writing; much appreciated.