Based on what we learned in the last update, I don't think it's possible for Himawari to be the game's first narrator.
As you pointed out before, we know that Himawari is the narrator of "What We'll See When We Get There" because she refers to herself as a flower that follows the sun, i.e. a sunflower.
She tells us stuff about her personal life.
She's actively growing, she lives in another world with her mom but her dad is gone, and she commutes to work at what I can only assume is the time travel factory. The most important part here is that outside of her appearance in Kumon-mi she's living the life of a normal teenage girl; she's not caught in a time loop, she's not an unaging supernatural entity, etc.
That's in contrast to what the narrator of "Every Day I Grow Some More" tells us about themselves:
The narrator has been trapped in the dark and very lonely for an indeterminate but probably extremely long period of time and has only been able to return to the light due to being assigned as Sensei's narrator. I don't see how this can be the same character as a fifteen-year-old girl growing up with her mother in another world.