If the two Way records can be joined up again, then Cimarron Hills, Colorado - CityStrides can continue to exist in CityStrides. I only have experience with the iD editor, and it’s not allowing me to join the two Way records. I gave it a try in JOSM, but was only successful in producing error messages.
The warning you were likely getting about “modifying an incomplete relation” was because the relation members weren’t sorted, so JOSM thought there was a bunch of unconnected ways. A quick sort fixed that up.
I was getting an error while trying to download the initial dataset I couldn’t even get to the point where I could attempt to join way records. It kept telling me that I was choosing too large of an area to download.
I combined those two ways into a single closed way. Hopefully that helps!
Edit: After looking at the boundaries a bit more, I went back and re-separated the ways where it shares a boundary with the neighboring city. The relation for Stansbury Park should no longer have the split across the city though.