As for the names of these relations, I do not care what they end up being. I chose "Missouri Route XX" since that's more consistent with MoDOT (previously routes were named "Missouri Highway XX", "Missouri State Supplemental Route XX", "MO XX", "SR XX", you name it), and it nicely matches the Wikipedia article titles (one such article is "Missouri Route M (Jefferson County)"). The latter carries less weight.

I mainly just wanted nicer alphanumeric sorting in JOSM. Feel free to change it to something else. As I state in my account bio, I value consistency over any notion of correctness.