A few things that occurred to me on reading this thread.
Registered co-writes can be clues to recent work - but they can't always be relied on as a guide to what may be on the next album. Sometimes artists register the songs that they've decided not to proceed with, but keep the ones still being actively considered as album tracks under wraps. However, it's often the case that several songs result from the same writing session - so the registrations can be interesting as an indication that x has been writing with y or z.
It is indeed the case that Carrie wrote earlier with Natalie Hemby - but I don't regard "Play On" as typical of Natalie's work. My hunch would be that Carrie herself put much of the input into that song . Like some other early Carrie co-writes (e.g Checotah, Crazy Dreams) it strikes me as mainly a personal song, which reflects feelings connected with her career. Also, it was used as one of the 'special' songs that Carrie likes to use as album closers. Its theme of carrying on in the face of pressures strikes me as a forerunner of Carrie's more mature recent song "Cry Pretty"
On the question of lead singles, I'd certainly endorse a choice like "Good Girl" - but, personally, I wouldn't lump that song together with "Cowboy Casanova". CC was certainly popular, and the visual treatment was interesting, but, in my opinion, it couldn't really be considered as a groundbreaking song, and going back to it stylistically might risk being seen as stereotyped. But, for me, GG was in another league. Musically and vocally it remains one of Carrie's most varied and boldest works to date. I would describe its conclusion as having parallels with a hypothetical 'Janis Joplin meets Stevie Ray Vaughan' - a real touch of Texas Blues influence, that helps the song as a whole to stand as one of Carrie's most complex and memorable achievements.