Oh, that's so relatable! A lot of things in my story were born from that line of thinking. "OK, Rue, but like... what if, what if!!" Then I try to tell myself no, then I fail to get the idea out of my head, then it ends up becoming canon.
A bit, yes! My Nanowrimo was a much earlier draft for the plot of the comic, and I think it helped a lot at the time! I'll admit, though, that a lot of my plotting thereafter was also done rambling on scrap paper during lectures in college... I could probably find one of them if I looked! They're just sheets of paper crammed from corner to corner with tiny words.
About the fluidity of the story, though! I had that problem, too, and what I did was just try to find a general central plot that I'm really happy with that could always remain a constant, but then give that central plot enough flexibility to change things around when I think it would improve things.
So, I always have the main plot to fall back on and use for structure, but I still have the freedom to adjust things here and there. Might not make for the most tight-knit story where everything, everything is planned, but it might be more your style?